Let's say you always start your invoice numbers with "INV" or "ORD", and you would like your autonumber field to do this automatically. No problem. In the Format propery of your autonumber field, you simply put
>"INV" OR >"ORD"
Now on all your lists, combos, reports, etc, the field will show up as INV34452 instead of 34452. This will work for any field, so you could do the same thing on a Text primary key value. So the user may enter 02577, but if you have >"INV" in the format property of that field, it will show up as INV02577.