As I got the answer from Qlik Tech, I reply by myself to the question.
The answer is below.
1. Since both these measure are from the Performance log, they are for the entire server, for the entire interval. Interval length is set in the Control Panel/Management Console for the log files. It is defaulted to 5 minutes, and can be set as low as 1 minute.
2. There are similar measure in the Session log, which would allow you to monitor specific sessions (a QlikView Server session is defined as one user + one document).
3. The selection counts increment by one for each specific selection, regardless of the number of values selected, e.g. if I select 3 products, and then select 12 months, it will count as 2 selections in the interval (assuming both selections took place during the same interval).
4. I'm not sure of the specific definition of Call, although I could guess that this means any communication between server and client (either way). I notice on my systems that there are many Calls, and much fewer selections. For example, I see over 20,000 Calls, and about 100 selections, probably because most of what I do is simply open documents and change sheets, without making many selections.