Squeegee blades are made of polyurethane in different hardnesses. A lower number is softer: the blade bends more and drives more ink through the mesh, which helps coverage on textured fabrics. A higher number is firmer: it shears ink cleanly across the mesh for a thinner, crisper deposit that suits fine detail and halftones. Ink data sheets commonly recommend blades in the 60s and 70s, and some blades bond layers of different hardness together.
Durometer works alongside blade sharpness, angle, pressure and stroke speed. Too much pressure or a soft, rounded edge can drive ink deep into the shirt so it looks dull, while too little leaves gaps and a rough surface. Water-based inks and plastisols each have preferred setups, listed by the ink makers.
You will not specify this on an order, but it helps explain why two printers can produce different results from the same art and the same blank. The mesh count and print feel guide covers the full set of press variables.