A compliance matrix lists every requirement pulled from the statement of work and sections L and M, then tracks exactly where in your proposal each one gets answered. Building one before you start writing keeps you from missing a requirement, which is one of the most common ways a technically strong proposal still gets marked nonresponsive. It also helps the evaluator find your answers, since a proposal organized around the matrix tends to mirror the order the solicitation asked for.

In a sentence

"Offerors are encouraged to submit a compliance matrix cross-referencing each requirement to the proposal page and section where it is addressed." Even when it is only encouraged and not required, building one anyway is one of the cheapest ways to catch a missed requirement before an evaluator does.