Learn English – a word to describe something that belongs exclusively to or is used only by one person or a group of people

I'm trying to find a word (or idiom or phrase) that describes something which is perceived as belonging to one person or group of people only. To contextualise this question I'll provide the paragraph in which I need to insert this word:

But while the community of administrative assistants the world over can thank the cast and creators behind The Devil Wears Prada for flagging their struggle to the public, they should also be disgruntling at the fact that the feature film reinforces the deterring notion that personal assistants are…[missing word or idiom]… to/of/for high-level executives.