Automatic color change.

On a multi-needle machine each needle carries a different thread, and a color change is the head sliding the needle case sideways so the next color comes over the fabric, with the old thread trimmed and locked on the way. Tajima's TMEZ single head, for one, is specified with 14 needles and automatic needle bar change on color changes within the design (Tajima TMEZ-SC1401, Hirsch specification). What it buys is unattended running: a twelve-color logo on a fifteen-needle machine never asks the operator anything, while the same file on a six-needle machine turns every color past the sixth into a manual re-thread across every head. Color count, not stitch count, is therefore what decides whether a multi-head can be left alone, and it is the number to read before quoting a design as machine time. Changes are not free either, since each one is head movement plus a trim and a tie-in, so a file sequenced into eight color blocks runs slower than the same design merged into four on identical hardware.