BANK cels are stock footage cels.  That means cels 
that are used repeatedly in each episode.  For 
example, Sailor Moon's attack sequences, her 
transformation scenes, etc..... are all examples 
of bank cels.  In general, I think they're higher 
quality cels and there's fewer of them since 
they're only painted once and reused for the 
whole season.  

A PAN cel is typically an oversized cel. It can be 
a cel that then camera pans over either 
horizontally or vertically for an extra long shot, 
hence the name, pan cel.

These are just my own interpretations of those two 
terms.  I'm sure others more knowledgeable than me 
could add to it.