Hello, I have an amtech staple gun. on the bottom it says 4-8mm for staples.

What does this mean? I assume this relates to the height of the staple so I can therefore get them either 4mm, 5mm, 6mm, 7mm, or 8mm. But then I dont know what width the staples need to be? I have already bought two sets of staples and neither fit, so I want to make sure I buy the correct ones this time. Thanks.