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 to this link 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.