i know this is old but for me this brings up several important contradictions within tiering, and somewhat different from what others have been (it seems to me) focused on.
~ im going to use the comments about victreebel and tentacruel as starting points, vic on one side being a pokemon that is clearly viable in 1u but is pretty bad in 2u, and tenta 'on the other' being almost completely unusable in 1u but having its own rank in 2u.
the two primary responses to this im seeing here are talking abt where to "draw the line" and talking abt wrap. the wrap discussion seems completely arbitrary yes these two pokemon both learn wrap but that really doesnt have anything to do with their strange tiering position and whatever people "think about" wrap has nothing to do with tiering practice (im not going to say anything further abt wrap here and i am only acknowledging it at all since so many comments were talking about it). as for "drawing the line" i think the concept contains numerous implicit assumptions which im going to go into here and if people are okay with the consequences of those assumptions thats fine but they should be openly acknowledged.
on the 'practice' side there is a central assumption throughout that if pokemon are ranked in a certain order in a given tier, that rank is automatically carried down to setting the next tier. in other words (using rby 1u as an example), if a pokemon that is B rank in 1u is permitted in 2u then all C and lower rank pokemon in 1u must also be permitted in 2u.
this is mostly arbitrary from a theoretical point of view because a pokemon can function better in one environment than another. but the contradiction is particularly prominent in rby 1u and 2u due to the qualitative centralization of 1u created mostly by chansey. (i mean centralization in the sense that being able to either threaten or stall chansey in some way is almost mandatory for a pokemon to have any 1u viability.)
even tho tenta has wrap, it cannot accomplish anything in a chansey environment because chansey forces it to wrap and then chansey can just switch right out, in 1u it switches out to tauros. this combined with tenta's lack of status moves and inability to function well when statused bc of being a wrap user ruins it in the paralysis centered 1u, despite having decent matchups against much of 1u. "power level" is a lil imprecise but to the extent that it is a usable concept tenta is a "generally powerful" pokemon in rby and while it has no place at all in 1u it is 'too' strong and 'over'centralizing in 2u (im not claiming anything about the possibility of banning it, i mean descriptively how it is so good in 2u that it has its own rank etc).
~ but (moving to the 'practice' side again) a lot of tenta's metadefining place in rby-2u must also be attributed to pokemon that would "likely" fit just fine in 2u but were 'automatically' banned from the tier (by tiering policy) due to their level of viability in 1u. im going to focus on jolteon because its the most unambiguous example but i dont mean to suggest that jolteon is the only "1u" pokemon this is true of. 1 twave is really valuable in 1u in general and specifically jolteon can outspeed and twave tauros (if jolt is unparalyzed) unlike zapdos; while twave is not as important in 2u and since jolteon has a horrible mu with dugtrio anyway its speed tier isnt really that much better than electabuzz's (im separating speed tier from crit ratio). 2 even tho jolteons coverage options are terrible they happen to have the perfect typing for 1u, most importantly double kick having the same bp as zapdos's drill peck vs chansey the tier's most important special wall, and the possibility of running pin missile for egg(/vic). in 2u the special walls are hypno and kadabra and jolt doesn't do that well against either, in addition to the ground types and tangela which are even worse matchups. (there are some other things like not being weak to blizzard in 1u but these are the two key aspects.)
even without any theorymonning i think it is very clear that the reasons why jolteon has any viability in 1u have to do with specific features of the 1u environment, mostly due to chansey, and it is clear that tiering is done in solely 'one direction', ie that the set of pokemon banned for 2u is determined solely based on their viability within the 1u environment, qualitative factors are basically not considered (meaning if pokemon A "has more viability" than pokemon B in 1u, the specific reasons why that is the case are not considered in terms of whether the pokemon is "considered 1u" and banned from lower tiers), and it is not permitted to argue for a "1u pokemon" to be allowed in 2u based on factors within the 2u metagame (as it exists at that time) which would be 'two directional' (or dialectical) tiering where you go back and review higher ranked tiers based on the development of lower ranked tiers.
if people wanted to re-tier rby 2u (in a way that is more theoretically sound and i think would also lead to a much better tier in practice, than something like just banning tenta would) two ideas i can think of are:
~ tier 2u based on a chansey-less 1u. chansey specifically changes 1u viability so much and since there have been various "tournaments" etc without chansey some people likely have at least a little experience with chansey-less 1u. maybe this idea sounds arbitrary but the reasoning is that there is no other pokemon in rby that is anything (at all) like chansey so the entire set of lower tiers shouldn't be impacted so much by one dominant 1u pokemon. it is 'like' how someone mentioned what if 1u was mewbers, that would obv dramatically change 1u tiering and therefore the starting point of what is allowed in 2u.
i dont really like this idea to be clear, tiering based on a relatively small set of competitive matches isnt good to say the least but i am mentioning it as a potential way to re-tier 2u without any change to the fundamentals of pp tiering practice.
~ this idea is arguably a 'partial form' of two-directional / dialectical tiering, to explicitly include qualitative aspects in the step 'between' having one tier set, and establishing the starting point for the next tier (rather than "drawing a line"). for example, in 1u u might separate the walls (ie the three twave users with instant recovery and maybe eggy goes in this group bc it plays more like a wall even tho it lacks recovery), the "pure" sleepers (jynx and prob gengar), the physical attackers (normals and goldon), and the mixed attackers / wallbreakers / special attackers with a potential way around chansey (zapdos/jolt, lapras, slowbro, maybe cloy dnite vic go here too). the four walls very clearly shouldnt be considered in any lower tier, while particularly the last group pokemon need to each be discussed individually with respect to whether their effectiveness in 1u is 'generalizable'. (this is very rough @ 'groupings' and more meant as initial ideas than a fully formed concrete analysis.) but the point is to evaluate the specific features of a pokémon's viability in 1u / in the given tier, and whether the viability is due to the pokemon's "general" strength or 'power level', in which case it is correct to ban that pokemon from all lower tiers, or if the viability is specific to the conditions of that tier in which case the pokemon should not be automatically banned from lower tiers.
and to reaffirm, i am using rby 1u/2u as the concrete basis for bringing up these points but i think they are important questions about tiering policy in general, rby 1u/2u is just where these issues have the biggest concrete effect due to how centralized rby 1u is around specific pokemon, mostly due to chansey
