To simplify, another use of the term is a delineation of how many points they score.
In a 12 team league for example, a RB1 is pretty much the RBs who finish 1-12 at year's end. A RB2 is the next 12 RBs.
Now that's not always perfect, but it helps explain what the other guy refers to with "tiers" if you don't already get that part. Sometimes you really have like eight RB1s, then there's a large drop off in scoring so you have like 16 RB2s, even though it is a 12 teamer.
Injuries mess with year end rankings too. I'd radther have a stud RB1 for 9 games than an RB2 who plays all 16 and outscores him on the season, but not on a per game basis.
I think its a big mistake to look at rb1-rb2 etc. in terms of 1-12, 13-24 etc. just because there happen to be 12 teams in your league. First of all, the rb's are never divided up evenly amongst the teams. Its not like each team gets a guy from each tier of 12. Many teams have 2 or more guys from the first 12 and other teams dont have any.
Secondly there is often a huge discrepency in scoring between say, the #2 guy and the #11. They arent equivalent caliber players and you cant expect the same results from both. So calling them both rb1's makes no sense and really doesnt help define what kind of player they are for your team
I think Pots had it correct in his post above when he defined them by their quality. I
would take it a step further though and say that it's a player's ppg (points per game) average that tells you if he's an rb1 or rb2 and so on, rather than his total points on the season. Simply beacuse some guys miss alot of games and cant be relied on as a true rb1, even if he performs really well when he does happen to play.
Try to read a little bit better, because I think I covered both things you find "wrong" within my post...
As to the underlined portion: If I drafted a perfect team, I'd have a QB1, two RB1s, two WR1s, a TE1, and another RB/WR1 for my flex. If that makes any sense. It doesn't matter if Arian Foster is my 2nd RB behind Ray Rice...they're both RB1s.
Edit: If you can see what I did, the bold correlates to bold, and the italics to italics.
Also - to explain this post a bit better: In my ideal roster, I am starting a RB1 in my RB1 spot and a RB1 in my RB2 spot. The terms are just like words, they have different meaning depending on context.