All sequences have key cels. But not all cels of 
a sequence are key cels. The first and the last 
of the sequence are going to be key cels, but 
only sporadic cels from the middle group will be 
key.

Key cels are drawn by key animators. You can tell 
they're key cels because their number is 
circled... A-1, A-30 END, etc.

Bank cels merely refer to a sequence that is 
recycled over multiple episodes... a good example 
would be henshin sequences or attack sequences 
from shows such as Sailormoon, where the same 
sequences are used over and over, regardless of 
episode. The attack or summoning sequences from 
something like Cardcaptor Sakura would not be 
considered a bank cel, because the attack or 
summoning would be uniquely animated each time it 
happened in the show. And things like eyecatches, 
OP, and END sequences are considered separate 
from bank cels as well-- they're important enough 
to be their own categories.

So figuring out if your cel is a bank cel depends 
on identifying how it was used within the 
animation itself. Then figuring out if your cel 
is a key cel depends on whether or not it was 
drawn by a key animator.