On her way: the moment she leaves for your door
On her way means she has left her starting point and is now travelling to you. From this point, a cancel costs more than it did before.
These three words mark a real moment in every booking. Before it, she is still at her starting point. After it, she is in the city, on the way to you. The thread may tell you when she leaves, and this is the moment it describes.
Why it matters for a cancel
With a deposit, a cancel after she is on her way means the whole deposit is kept, unless someone else books the hour. Without a deposit, a cancel after this moment costs the part of the price that her journey stands for. Before this moment, both costs are lower. So if you are unsure about a booking, say so early.
Why it matters for payment
Any figure in another currency must be written in the thread before she is on her way. The same is true of a same-day booking where you confirm that you want the session straight away.
What to do now
Very little. Keep your phone close, with the sound on. Have the balance counted. If your building has a street door, be ready to go down. The arrival window tells you when to expect the knock.
How you know
The house may write a short line in the thread when she sets out. If no line comes, the arrival window still tells you everything you need, because it counts from your confirmation and not from her departure.
A new room number on the way
If your room changes after she has set out, write the new number in the thread at once. A move within the same hotel costs nothing, as long as it is written before she reaches the lobby.