Squirrel is a disparaging term used by Hubbard and Scientologists for anyone who does not apply Scientology standardly. That is fine in a simplistic sense, but in reality it is very hard to accurately define what is standard Scientology. There are various definitions in the Technical Dictionary, but these fall far short of being adequate. See, for example, this article  explaining the shortcomings of the term.
Inside the CofS, the term may be used to describe an auditor who made one error in auditing; or someone who has routinely misapplied technical data for years. It is not consistent. Of course the CofS refers to Freezoners as squirrels, but individual FZers may also refer to other FZers as squirrels because they disagree as to what is "standard" or not.