Confirmation: the yes that fixes the booking
A confirmation is the point where you and the house agree to one and the same booking. Before it, nothing is held. After it, the hour is yours.
To confirm means to make something certain. In a booking, the confirmation is the point where the details stop being a question and become an agreement. The reply names who is free, the price and the window. You answer yes. That yes is the confirmation.
What your yes should hold
One word is enough, but a short line is safer. Repeat the day, the date and the hour: yes, Tuesday 3 November, 20:30. If any detail is different from the reply, the difference shows at once, and it can be put right before anything is fixed.
What happens after it
If there is a deposit, the payment details come in the same thread. For a same-day booking, the arrival window starts from the moment of confirmation, and she arrives forty to sixty minutes later in central Amsterdam.
Before it
Nothing is fixed. You can ask questions, change the hour or change your mind, and it costs nothing. So take your time before the yes. A question asked before it is always better than a change after it. The words in the reply page explains the rest of the thread.
Who sends the last word
The house replies first, with a therapist, a figure and a window. Your yes comes after it. Until you send that yes, the reply is only an offer, and the hour can still go to another guest who answers before you.
A confirmation for later today
For a same-day booking, the house also asks you to write that you want the session to happen straight away. That short line belongs in the same message as your yes, or just after it.