Nursing/Maternity clips, take two straps, the measurements given are the size of those two straps widths.
The first number is the width of the plush strap that attaches to the bra, the second is the width of the Original bra strap. Measure the width of your bra strap and choose a fitting with the second number close to this width.
Use the first number to determine the Plush elastic you need to buy (you may be making an inner cup and not using elastic.
The SECOND number is the shoulder strap width.