Primarily, the zippers offered here are rarer, larger widths than normal every-day zippers. The gauge numbers translate directly into the width of the teeth across in mm: so #10 equals 10mm wide, #20 equals 20mm wide, and a #30 equals 30mm wide – with the measurements taken when both halves are zipped together. This is the standard measurement term for all zippers, so even smaller ones will use this format: a zipper labeled #5 being 5mm wide, for instance.
(Only the raised portion of the teeth themselves are indicated by this width, and not the slider/pulls nor the end stops – which are usually a bit wider. Also the fabric tape is not directly measured and may slightly vary.)