The Vivado tools track two different types of placement:
- Elements placed by the user (shown in orange) are Fixed.
- Fixed logic is stored in the XDC.
- Fixed logic normally has a LOC constraint and might have a BEL constraint.
- Elements placed by the tool (shown in blue) are Unfixed.
In the following figure, the I/O and Block RAM placement is Fixed. The slice logic is Unfixed.
Figure 1. Fixed and Unfixed Placement