Underlay.

Underlay is the layer an embroidery machine stitches before the cover stitches, tying the fabric to the backing and giving the top thread something to sit on. Wilcom's manual is blunt about the alternative: without underlay the embroidery lies flat on the fabric, which often shows through, and the cover stitches sink into anything soft (Wilcom EmbroideryStudio help). It also flattens nap, so a logo on fleece or pique reads as a logo instead of thread lost in pile. The mistake shops make is answering a thin sew-out by raising density, when the fix is underlay: more thread through the same holes perforates the cloth and pulls it harder. Type follows shape, and the manual is specific, with a centre run stabilising a narrow column of 2-3 mm while large shapes want an edge run plus a zigzag or tatami layer (run stitch underlay settings). Underlay adds stitch count, so it adds machine minutes, which is why it is the first thing trimmed on a price-driven quote and the first thing missing when the sample comes back puckered.