OU = "overused", which is just a short way of saying
"These are pokemon that are used a lot."
Notice, that I did not say
"These are good pokemon", or
"These are powerful battling pokemon", or even
"These are pokemon that you should use if you want to win".
It simply expresses that many people use these pokemon in competitive battles. Maybe the people using these pokemon are stupid, or inept, or whatever. Or maybe they are geniuses. It doesn't matter what they are, or why they are using certain pokemon. When we label a group of pokemon as "OU", we are simply expressing that those pokemon are currently the most widely used pokemon in standard ladder play.
UU = "underused", which is just a way of saying
"These are pokemon that don't get used very often on the standard ladder".
Notice that I did not say
"These are crappy pokemon", or
"These pokemon can't win in OU", or
"These pokemon are roughly at the same power level as Steelix, Swellow, and Hitmontop".
It simply expresses that these pokemon aren't seen very often on the standard ladder FOR WHATEVER REASON. Maybe there are other pokemon that often put these pokemon at a disadvantage in standard play. Maybe these pokemon are "outclassed" by better options in standard play. Maybe most people in the community just really hate the pokemon's game sprite and refuse to put it on their teams. Who knows? The fact is -- it doesn't matter WHY they aren't used often in standard play. By tagging a pokemon as "UU", we are simply indicating the cold, hard FACT that the pokemon statistically does not appear as often as some other pokemon.
So, the basic determination that a pokemon is "OU" or "UU", is a simple matter of measuring USAGE. That's why the tiers end in the letter "U", which stands for "used", as in "usage". You can question the math formula for measuring usage, and you can question the numeric boundaries that have been set for "Used a lot" (aka "OU") and "Not used as much" (aka "UU"). But, do not confuse that to be a discussion of
"How powerful should a pokemon be, in order to classify it as OU or UU."
Measuring usage has nothing to do with measuring usefulness.
After we have mathematically grouped pokemon according to how much they are used -- then we look and see if any of the pokemon in a given group, are too strong to be played with the other pokemon in that usage group. That process is what we call "suspect testing". "Suspect" is the short term for:
"We suspect this pokemon may or may not be too powerful within this group. We don't know for sure, but there is general consensus that it is a question (ie. a suspicion) worth investigating further."
We then go through a series of objective and subjective processes to try and answer the question (confirm or deny our suspicion). We call this "Testing the suspect", or "Suspect testing".
In the case of Cresselia and Porygon-Z, the objective measurement of their usage on the Standard ladder determined that they are NOT used very often in standard play. This fact cannot be argued. Even if you dislike the formula, you cannot argue that these two pokemon, according to our mathematical definition, are not seen very often on the Standard ladder. Therefore, they ARE "not used very much" right now. They are "UU".
Now we are left to determine if they are too powerful to be played with other UU pokemon. At Smogon, we prefer to act on information gleaned from actual battling experience -- not on theoretical musings by so-called "pokemon experts". You are welcome to disagree with us, but we firmly stand behind our assertion that real pokemon battling is far superior to sitting back and imagining how you think a battle will play out in your head. Theories are fine for discussion, but any decent theory should be able to be proven through practical application. So, when it comes to testing pokemon that have been mathematically recategorized, we feel that some level of actual battling should take place.
That's what we are doing with Cresselia and Porygon-Z. We are going to play with them in the tier that they have been factually proven to be part of (UU). If, after playing them, there is a consensus of people that battle frequently and are successful in that tier, that Cresselia or Porygon-Z have a detrimental effect on the competitive balance of the tier -- then we may ban them from play within the tier.
I don't know why so many people seem to have a hard time with these concepts. But every time I see someone post
"I can't believe Smogon has decided to make Cresselia UU!", it makes me cringe. Statements like this imply that we are somehow forcing a pokemon to be used or not used in standard play. When, in fact, all we are doing is measuring usage. Nothing more.