Thank you very much for replying.
You are all right that in the given example a fully defined definition would require a position (or profile) tolerance which includes at A and at least C datums. An interesting question is if including B datum would change anything in the measuring process and results.
But I just wanted to check if there is an option of giving Parallelism of a plane to an axis, a requirement that is relevant in another part I design.
I didn't encounter such a case, so I wanted to make sure it's allowed by the standards.