If you want to unlock all the extras you will need to receive friend vouchers which turn into friend credits.
Credits are the main currency that you use to unlock the extras like art concept, soundtrack, and cool extras for your ship, credits also come in the form of red, blue, and yellow, a variety of combinations of all these credits will allow you to unlock all the bonuses I've mentioned.
By the way the extras are listed in the extras menu before you start your savegame. Some of them have a

on them but I think those are just for the diagrams (or is it dioramas? hm), but I haven't unlocked those yet, damn those yellow credits sure are scarce.