Guiding inputs

If this field is set to true, the server must not execute any side effects and only perform implementation-specific validation consistent with the full request.

some API methods cannot easily be idempotent, such as creating a resource, and there is a need to avoid unnecessary duplication

This means that the server must ignore the presence of output only fields and any indication of it.

... without referring to external documentation, an API user does not know what data to provide to operations that take input.