When is it necessary to use a cap during highlighting?

The use of a cap is specifically associated with the cap highlighting technique, which is a method where a plastic cap with holes is placed over the head, and strands of hair are pulled through these holes using a hook. This technique allows for controlled and precise application of highlights. The cap helps to create a neat and organized look, ensuring that the color is evenly distributed and that specific strands are highlighted while others remain untouched.

In this context, the other options do not accurately reflect the specific circumstances under which a cap is needed. For example, using a cap during every type of hair coloring is not necessary, as many coloring techniques do not involve a cap at all. Additionally, the use of a cap is not limited to curly hair or required for every hair treatment; many procedures utilize other techniques that do not incorporate a cap. Understanding these distinctions helps clarify when and why a cap is a valuable tool in the highlighting process.