Bank cel: A cel from a sequence that is used more 
than once.  These would include the SailorMoon 
transformation sequences.  Another example is the 
fight sequences in Gundam Wing.  Some action 
sequences are exactly the same from episode to 
episode but with different backgrounds to make it 
seem different.

Book cel setup: A background that covers part of 
the FRONT of the cel.  An example would be a 
cutout tree background placed over a cel to cover 
part of a character.