If PMO_FILE_END is set in a PMO_LOADING_FILE packet, LFS has reached the end of a layout file which it is loading. The added objects will then be optimised.
When objects are moved or modified in the layout editor, two IS_AXM packets are sent. A PMO_DEL_OBJECTS followed by a PMO_ADD_OBJECTS. In this case the flag PMO_MOVE_MODIFY is set in the PMOFlags byte of both packets.
If you send an IS_AXM with PMOAction of PMO_SELECTION it is possible for it to be either a selection of real objects (as if the user selected several objects while holding the CTRL key) or a clipboard selection (as if the user pressed CTRL+C after selecting objects). Clipboard is the default selection mode. A real selection can be set by using the PMO_SELECTION_REAL bit in the PMOFlags byte.
If you send an IS_AXM with PMOAction of PMO_ADD_OBJECTS you may wish to set the UCID to one of the guest connections (for example if that user's action caused the objects to be added). In this case some validity checks are done on the guest's computer which may report "invalid position" or "intersecting object" and delete the objects. This can be avoided by setting the PMO_AVOID_CHECK bit.