SYSML16 — Allow a Requirement to be contained on Any Diagram

Strict implementation of a Requirement, which is based on a Class, restricts the expected usage of a requirement on diagrams that do not allow the visualization of a class. Not all tools enforce this, but those that do (MD), restrict the desired approaches to addressing requirements traceability and communication.

SysML has no strict constraints about diagram content

Section "16.3.1.4 Requirements on Other Diagrams" says "Requirements can also be represented on other diagrams to show their relationship to other model elements." If a tool enforces the usage of requirements on a diagram, it is a tool issue.