I would not translate the German term directly. I don't think it makes much sense in English.

I suggest "priority for collective regulations" or "priority for collective rules"

In this context "vorbehalt" is essentially a "proviso"... rather than a "reservation". What it means is that the individual contract is subject to the provisions of later workplace agreements (Betriebsvereinbarungen). In other words these collective rules take priority even if these contain provisions less advantageous to the employee than his/her individual contract.

If the audience is British it's important not to use "collective agreement" for Betriebsvereinbarung as "collective agreement" is the English law term for a Tarifvertrag.