ISO standard - A parallelism of a plane to an axis 4

I would say yes. You can require a parallelism of indicated plane with the shown axis.
I am just wondering if the orientation plane indicator will provide more clarification to your requirements (as an addition to your original callout) to improve the clarity and stabilize the tolerance zone

Just to make sure you understand correctly.
Shown callout (with or without my orientation plane idea) will never locate the such said plane. Will only orient it.
So you are currently missing the location control: position to A

Agree with greenimi:

Position of 0.2 w.r.t. |A|C| will also cover straightness, flatness and parallelism.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

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.

I don't think just adding B will do anything. Position is probably better but I think you can change this to angularity and then use A&C, or if you are more interested in the udim'd other slot you can use that instead of C

I'm kind of later to the party here but the answer is yes, you can do it. There is a hidden gem of a standard ASME Y14.5.1M-1994 with seeming abstruse name "Mathematical Definition
of Dimensioning and Tolerancing Principles". If you go all the way to the back, they have all kinds of "off the beaten path" examples there. Here is one for your case.

I am just curious why we are talking about ASME staff when the OP clearly stated that his question is/was about ISO GPS?
Why we are trying to use example from a different system (ASME) to support an ISO GPS question?
And I agree with Diametrix final answer ("YES, you can do it"), but with his/her overall explanation....................................

Thank you all for your good advices and contributing information!!! This is a great forum.