Fixed and Unfixed apply to placed cells. They describe the way in which the Vivado tools view placed cells in the design.
For more information about Fixed and Unfixed Cells, refer 'Modifying Placement' in the Vivado Design Suite User Guide: Implementation (UG904).
Recommended: After the I/Os are placed, use a hierarchical Pblock
floorplan as a starting point for user-controlled placement. Hand placing logic should
be used when Pblocks have been found not to work.