In-room is the hotel industry's term for anything brought up to the guest; here it names a session held in the guest's own room, which every reservation is.
The hotel directory on the desk already uses the phrase: in-room dining, in-room safe, in-room entertainment. Each means the service comes to the room rather than the guest going to it. When the house says in-room, it means the same thing, and because there is no premises of any kind, it describes every reservation taken.
A hotel spa is a department with a floor, a reception desk, a menu and hours. In Amsterdam those hours are often shorter than visitors assume, with a last appointment well before ten. An in-room reservation keeps none of that structure. It begins at the hour the guest names, including one in the morning; it runs to the length he chose; and there is no one waiting outside for the room when it ends. The comparison is drawn in detail in hotel spa or in-room, and the journal note the spa closes at ten looks at the clock in particular.
Less than guests expect. The bed serves as the working surface in almost every hotel room, and her own linens go over it, so the hotel's sheets stay as they were. Warmth matters more than anything else in preparing, since air conditioning set for a summer afternoon is too cool for anyone lying still. A lamp rather than the ceiling light, a clear path around the bed and a bathroom free to use complete the list. Towels are hers, not the hotel's.
A compact double on a side street and a corner suite with a canal view are priced identically on the ladder, and neither carries a surcharge. What a larger room changes is space, not cost. For the practical detail of what arrives and what is expected, the guide what an in-room massage includes goes further.
Tonight
Name the room, the hour and how long you would like her there. The reply comes back in about ten minutes with who is free.