Those are not in the crew check list. They are in the Shuttle Launch Countdown procedure, OMI S0007

Both refer to a series of orbital maneuvers which allowed the crew to take photos of the external tank and the underside of the orbiter, for later analysis by ground engineers. The "plus x" is s translation along the X axis, and the "pitch" is a rotation along the Y axis. The Y axis rotation was commonly called the RPM or rendezvous pitch maneuver, since it was done in before docking with the ISS.

The Plus X, after MECO and tank sep is when they maneuver the shuttle to allow the umbilical well cameras to capture imagery of the tank, immediately after separation. The pitch, after MECO and tank sep is to allow crew members to take photos of the tank. The Rendezvous Pitch Maneuver (RPM), allows astronauts on station to capture images of the shuttles underside, prior to docking with the ISS.

Yes but what you're all missing here is that the PLUS X maneuver was initially just a dump of excess OMS fuel to give the Orbiter an extra boost to its orbit target. When it was realized that an RCS pitch maneuver could be accomplished in parallel to get photos of the just recently jettisoned ET, this was when the "PLUS X and PITCH" actions were added to the Shuttle's timeline post MECO.

What those are, are last minute opportunities for PASS and BFS uplink loads late in the count. What this means is that the NTD (Nasa Test Director at the KSC Firing Room) has a point in the KSC Launch Procedure where they ask Houston for any final flight dynamic, trajectory or other primary/backup computer updates, PASS being the primary, BFS being the backup. In almost all cases, the Houston Flight Director will report "no changes" or a "not perform" to the NTD for this step.