18.104.22.168. Using Low-Level I/O Primitives
You can alternatively enter I/O pin assignments using low-level I/O primitives. You can assign pin locations, I/O standards, drive strengths, slew rates, and on-chip termination (OCT) value assignments. You can also use low-level differential I/O primitives to define both positive and negative pins of a differential pair in the HDL code for your design.
Primitive-based assignments do not appear in the Pin Planner until after you perform a full compilation and back-annotate pin assignments (Assignments > Back Annotate Assignments).