populationFollow

Those are progression servers that only progress after a certain time period and a majority (or some standard) of voting (I don't know the specifics, as I haven't participated in the voting as of yet). This means, that only certain expansions are available at any given time, and they open new ones up in the same sequence they originally came out, but on their own time-table and based on the voting. Mostly the two servers have been fairly close to one another on new expansions opening but, but not always at the exact same time.

I think there's more information on the Sony forums somewhere, but I don't remember where exactly off-hand.

Yther Ore.