What is the difference between a tech pack and a spec sheet?

A spec sheet describes the product, a tech pack describes how to make it. A spec sheet lists measurements, materials and colours. A tech pack adds the construction detail a line needs: seam types, stitch density, cup construction, elastic tension, labelling position, packing method and the acceptance standard.

A spec sheet is enough to quote from. It is not enough to produce from, and the gap between the two is where most first samples fail.