Rule 16.2 states that it applies between boats on opposite tacks on a beat to windward. Rule 18.1(a), states that rule 18.1(a)  does not apply between boats on opposite tacks on a beat to windward.

For the purposes of these rules, when are boats on opposite tacks on a beat to windward?

Two boats on opposite tacks are on a beat to windward when, after their starting signal,

Examples
In each of the situations 1 to 6 that follow, the boats shown are on opposite tacks on a beat to windward and are either below, on or overstanding a closehauled  layline to the next mark of their course. This mark may be a rounding mark, a passing or limit mark, a gate mark or a finishing mark.

Situations 5 and 6 illustrate two boats on a beat to windward in the zone of a mark to be left to port. In situation 5 condition (1) applies and in situation 6 condition (2) applies.

In situation 7 each pair of boats below is on a beat to windward and the starboard-tack boats are each subject to rule 16.2.

World Sailing 2021