File numbers are at the discretion of the originator. The listing above posted by P Kaye is derived from the CF File Classification Manual so the numbers to the left of the hyphen are "national". After the hyphen "0" would be "Policy" and "1" would be "General". Beyond that point for the most part is up to each unit, provided they consult with the publication first.
In your case, I would recommend that you use your PRI Number (eg: 111 222 333 (Pers)) as this is basically a personal matter and you will naturally want a copy of that memo on your personnel records, vice unit files.
If that doesn't work for you then you can use 6006-1 (Civ Pers - Comp and Benefits) because Leave is a benefit.
Enjoy.