You get the new Realtracks and you get a copy of RealBand. RealBand will read all your BIAB styles from your BIAB folder, all of your existing RealTracks and RealDrums, and it will give you the ability to create songs in sequencer environment. That includes the BIAB chord grid, BIAB styles. Plus you can regenerate BIAB individual tracks and even a single measure within that track, without having to regenerate the rest of the tracks. Plus all the other features of PTPA except for TC-Helicon features. Of course, the RealTracks you get with the Plus Pak will also work with your existing BIAB. Remember, PTPA currently does not support RealTracks. But with RealBand, as long as you have a chord progression defined in the chord grid, you can generate RealTracks by simply right clicking on a track and selecting the appropriate RealInstrument. It's actually easier to just do it than to try to describe it. RealBand will directly read in BIAB MGU and SGU files, as well as SEQ files, WAV, MP3, WMA, and MIDI files. There's a lot of power in this combined package.
RealBand, however, is more like PTPA with BIAB features. Other than not having TC-Helicon, it has (as best as I can tell) all the rest of the PTPA features, but adds a lot (but not all) BIAB features. You won't get melodist or soloist or harmonist or embellisher or a number of other features. But you can still do those in BIAB, and once they are stored in the BIAB tracks and saved to a file, you can open it in RealBand and continue workng there.
Hope that helps.