In-room describes a session delivered inside the client's own hotel room or apartment, the setting every booking with this house takes place in.
Hotels use the phrase in-room for anything brought to the guest rather than taken downstairs: dining, a film, a treatment. In this trade it describes a massage that happens in the room where the client is staying. Every booking here is in-room by definition, since the house has no premises of its own.
The phrase is most useful as a contrast with the hotel spa. A spa sits on another floor, keeps opening hours, books in fixed slots and works to a published treatment menu. An in-room session keeps none of those constraints. It can begin at two in the morning, it can run for three hours, and it follows the technique the client booked rather than whatever the spa has on its list. The spa and in-room comparison sets the two side by side in more detail.
Because the client provides the setting, a few minutes of preparation help. Warmth matters more than people expect; hotel air conditioning in summer and a cold apartment in winter both work against a long session, so set the thermostat before the therapist arrives. Clear the bed or a wide stretch of floor. Keep a couple of large towels to hand, and leave the bathroom usable. Nothing else is required: the therapist brings what the technique needs.
It also helps to have the room number correct in the booking thread and to know how the building handles visitors after dark. Some hotels ask guests to come down to the lobby; others let a visitor ride the lift straight up. Either is fine, and the office will ask where there is any doubt.
In-room does not mean a longer or shorter session, and it carries no surcharge. The rates printed in each therapist's profile apply unchanged whether the room is a single on a side street or a suite with a canal view. What the setting does change is privacy: nobody else sees the client arrive, wait or leave, because he does not go anywhere.