A good example is two boats approaching a leeward mark to be left to port. Both are on port, and an overlap exists between the two boats.
The leeward boat is slightly ahead, with the bow of the windward boat even with the leeward boat's shrouds.
The leeward boat reaches the zone first, which is defined as three of her boat lengths.

18.2(b) says:
If boats are overlapped when the first of them reaches the zone,
the outside boat at that moment shall thereafter give the inside
boat mark-room. If a boat is clear ahead when she reaches the
zone, the boat clear astern at that moment shall thereafter give
her mark-room.

In this case then, the outside boat, despite reaching the zone first, owes the inside boat mark room.