It's a 'key cel'.  The number is rarely circled 
on the cel, so the douga is the best indicator.  
Sadly, not all cels come with douga ;_;  If it is 
multilayer, then the layers with douga that have 
numbers circled are the key parts.  Also, first 
in sequence cels (A1, B1, C1) and End cels are 
also key.

Enjoy your cel!
Keys