Not sure what they ment, but I interpret “in your possession” as “you can control the use of the computer”. In which case I think it’s one of:
- You own it.
- Only you are using this computer (in OP’s case only during the period he is sitting before it).
- You are supervising this computer (can decide who is using it and who isn’t).
In practice, yes. You can do no harm to Steinberg by installing Cubase on any computer you ever use and I doubt they would sue you for doing so, but legally “Terms & Conditions” limits your freedom on installing Cubase to any computer. And further nitpicking: I’m not legally limited by “Terms & Conditions”, because they are not provided in any of the official languages of my country. And law in my country requires this for them to apply 
.