The numbering system in Shelterluv is automatic and will assign a number starting with your shelter's prefix, then A for animal, then the next available sequential number, XYZ-A-1234, for example.
You can use the Previous Shelter ID # field to assign another ID such as your own that is searchable but this is an open text field and will not auto-populate a number for you. The Previous Shelter ID # can be found at the bottom left hand of each animal's profile, can be configured to appear on your intake forms, and is searchable when you click the small black arrow in the search bar.