The ability for people to work on a different site than their usual one is sometimes called "nomadisme", so such desks are sometimes called "bureaux nomades" / "postes de travail nomades" / "positions de travail nomades".
Note that "nomadisme" itself is also sometimes used to include any form of work outside of their usual work site, including at home. But a "position de travail nomades" is of course on a site.
Note that there is a lot of confusion around the words for the "desk" itself, regardless of its use:
- "Bureau" can be the desk itself, or it can be the office (room). It is equivalent if there is only on desk in one room, but quite different in shared offices.
- "Poste de travail" can be the desk, or it can be a computer (either a desktop PC at the desk, or the user's laptop)
- "Position de travail" is the least ambiguous. Often abbreviated "PDT".
A given desk can have any of the following uses:
- "Fixe", "Attribué", "Assigné" for desks which are permanently assigned to a given user
- "Flex", "Flexible", "Partagé" for desks which are not permanently assigned. It is usually used in a context where regular users on a given site do not have a permanently assigned desk and can pick (or be temporarily assigned) a desk on a day-to-day basis in their usual work site, but depending on the company's organisation and policies, it may be open to visiting users from other sites as well.
- "De passage", "Nomade", "Visiteurs", "Mobilité" are desks for users from other sites who come for a few hours or a few days at a time. Sometimes the english "Hot desk" is used as well.
That gives you quite a number of combinations :-)