Sure.
Simple: test some number of cards (including 'all') matching a particular set of criteria, with whatever test type you like. No effort made to optimize learning intervals or whatever, you just tell it what + how to test you and it does. So an example scenario would be that you're practicing for a test and you want to review all of the vocabulary for it, or a random subset of that.
Drill: similar, but in this case the criteria can be 'things that happened in other profiles', it can draw from more than one, and it can even do stuff like mark the cards correct/incorrect/reviewed in their original profiles when they're done. An example scenario here would be 'review all of the cards I answered incorrectly yesterday, with the same test types.'
Learning: this is kind of our take on SRS, it synthesizes some ideas from 2.0/3.0 (like 'score-based subject selection') with some ideas from other SRS systems plus some stuff that we came up with just for 4.0. Basically, it takes cards through a progression of 'stages' of increasing intervals / more challenging test types, with incorrect answers dropped back to a shorter interval and the number of new cards added per day managed (there's even a clever little 'new card queue' system so you don't have to keep going back and picking a new category or whatever when you run out of new cards).
By default it only uses spaced repetition for learning; once a card is in 'review' all review cards are treated more or less the same, they're not tested on any sort of rigorous set schedule but brought up again in approximate order of when they were last reviewed. (the thinking here being that since you're going to encounter words 'in the wild' anyway, even if an algorithm could predict memory decay precisely it wouldn't have any way to account for those other exposures) But we do also offer an option for a SuperMemo/Anki style SRS with easiness levels and so on.
(there's also the "SM2" profile, which was basically taking the Anki/SuperMemo algorithm emulator we wrote for our Anki importer and separating it from all of the added baggage of decks / models / etc (which get their own special interface if you actually import Anki cards); if you're wary of our take on 'learning' that's another option)
If you restore a database exported from the old app, it'll wipe out your other flashcards but will also give you another legacy profile. This should also happen if you import an XML (not text) file exported from the old app, if you want to be able to bring in legacy cards without replacing other stuff. And you can certainly use them side by side in parallel, yes - internally it's just another test profile (technically one parent legacy profile and a child profile for each score file), it doesn't affect any part of the flashcard system outside of that.
