Landside: the side of the airport a session can reach
Landside is the part of the airport before passport control and security. Only hotel rooms landside can be reached, so only they can host a session.
Airports divide into two halves. Airside is everything behind security and passport control: the gates, the lounges, the shops for departing passengers, and any rest facilities built inside that secure area. Landside is everything else: the arrivals hall, the station, the car parks, and the hotels a traveller can walk or take a shuttle to from the street.
Why it matters
A therapist is a visitor. She can reach any room that a visitor can reach, which means any room landside. She cannot pass security to reach a room airside, however close it is to the gate. So a session is only possible in a room on the landside of the airport.
Which hotels are landside
Every hotel in Schiphol-Rijk and Schiphol-Oost is landside, as are the hotels on the airport grounds that are reached from the forecourt or by shuttle. The distinction matters mainly for passengers on a connection who never leave the secure area. If you are staying in a rest facility behind security, a session is not possible there.
Connections
A passenger with a long connection who wants a session needs to pass through passport control, leave the secure area and check into a landside hotel, then come back through security for the onward flight. That takes time at both ends, and it works only on a connection long enough to absorb it. The journal note on a room booked until four works through the arithmetic.
The short test
If you can walk out of your hotel's front door onto a road, it is landside. If you would need a boarding pass to reach it, it is not.