The TS_FRAME_ACKNOWLEDGE_PDU structure is a client-to-server PDU sent to the server whenever the client receives a Frame Marker Command ([MS-RDPBCGR] section 2.2.9.2.3) with the frameAction field set to SURFACECMD_FRAMEACTION_END (0x0001) and it has finished processing this particular frame (that is, the surface bits have been rendered on the screen).