What is 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.


The word reserved (meaning set aside for specific people in this context) works here.

"... the deterring notion that personal assistants are reserved for high-level executives."

ODO:

reserved adjective

2. kept by special arrangement for some person: a reserved seat.


Alternatively 'prerogative' may be what you are looking for

A right or privilege exclusive to a particular individual or class:
‘in some countries, higher education is predominantly the prerogative of the rich’

Per your example

the feature film reinforces the deterring notion that personal assistants are the prerogative of high-level executives.

This can also be intensified by the use of 'sole'

the feature film reinforces the deterring notion that personal assistants are the sole prerogative of high-level executives.


Peculiar to
perhaps not the most common usage of 'peculiar' but i think it would work in the example.

2.1 peculiar to Belonging exclusively to:

  • ‘some languages are peculiar to one region’

  • ‘ I'm not sure if all people do this, or whether it's something
    peculiar to my family.’

In your example:

the feature film reinforces the deterring notion that personal assistants are peculiar to high-level executives."

Though I might rephrase to something like

the feature film reinforces the deterring notion that having personal assistants is peculiar to high-level executives."


Exclusive (to) works particularly well here:

exclusive adjective

(postpositive) foll by to. limited (to); found only (in):

this model is exclusive to Harrods

As well as meaning something belongs to a particular place, group or individual, it connotes a degree of privilege and status often associated with the group under discussion.

Limited (to) and reserved (for) have similar connotations.


More generally, idiosyncratic (to) and peculiar (to) mean exactly what you intend:

idiosyncratic adjective

[…] something peculiar to an individual

peculiar adjective

  1. belonging characteristically (usually followed by to):

    an expression peculiar to Canadians.

  2. belonging exclusively to some person, group, or thing:

    the peculiar properties of a drug.