Cold peel or warm peel specifies when you should peel off the liner after using your heat source.
Both approaches are applicable, you may use both and decide which one best fits your needs.
Hot Peel: remove the liner while your project is warm to the touch, but not hot (the plastic will be very hot at first and can burn you). This is when removing the liner will be easiest. If you allow it to cool completely, it will just take a little more effort to remove the Pretreatsheet/film.
Cool Peel: allow your print to cool completely before you remove the sheet/film. If you remove the sheet /film while it’s still warm or hot, you risk your project warping, bubbling, peeling, or stretching.