How can P be required to keep clear of S AND S be required to give P mark-room? If you look at the definitions, those appear (to me) to be mutually exclusive requirements.

Keep Clear: A boat keeps clear of a right-of-way boat if the right-of-way boat can sail her course with no need to take avoiding action.

Mark Room: Room for a boat to leave a mark on the required side. Also,
(a) room to sail to the mark when her proper course is to sail close to it, and
(b) room to round the mark as necessary to sail the course

Room: The space a boat needs in the existing conditions, including space to comply with her obligations under the rules of Part 2 and rule 31, while manoeuvring promptly in a seamanlike way.

Thus, for P to KEEP CLEAR of S, then it means that S has to be able to sail her course without needing to take action to avoid P. This of course is not possible if at the same time S must maneuver to give P mark room.

sm