ASC 2 means biometrics, ASC 3 means biometric and FP and ASC 1 I think is only FP.
Many of the codes differs from each other because it depends in what status you get in or you are and when you apply too.
It is my understanding the biometrics consist in a digital picture and a signature (with specific digital features).
On the Notice of Action appears a different ASC for each application it is my understanding...
FBI checks are two types: fingerprinting check and name check.
The first one is the FP check, is made within 48 hours. Then your EAD should be approved. It is very interesting the one who received an EAD with "FP not available", your AOS is base in what? employment?
The name check is done during your AOS is in process, some people have it done by the interview time or some are stuck until the name got clearance from FBI. Since 9/11 if you don't have a name clearance you don't get your GC card even an officer (in an interview or other situation) tell you that everything is in rule.
Good luck,