Most blanks are piece dyed: the fabric is dyed in rolls, then cut and sewn. Garment-dyed styles are sewn from PFD fabric and dyed afterward in large machines. Color settles slightly differently into seams, collars and hems, creating the lived-in, vintage look these shirts are known for. The hot dye bath also takes care of much of the shrinkage before you buy.
The catch is variation. Shades can shift between dye lots and even within one lot, and sizing can be less consistent than on piece-dyed blanks. For printing, that means the base color under a print can vary across a run, and discharge or water-based inks need testing on the actual lot.
If you reorder, expect a close match rather than an exact one, and buy enough of a color at once when consistency matters. The garment-dyed blanks guide and the Comfort Colors 1717 page cover the details.