In some contests it is necessary to maintain sequential serial numbers for both stations (not per station/band) in the multi/single category. If you are not careful you may give out a serial number that is taken by the other station(s) if you do not enter the QSO quick enough!
To overcome this, it is recommended on the mult station(s) to type the call/double-click the spot of the station you are trying to work, and as soon as you hear him come back to you, enter the QSO immediately by entering serial number 0 (zero). Then simply press the up arrow on the keyboard to the QSO and use the spacebar to move across to the serial number field and enter the correct serial number given by the other party.
If you were not quick enough to press the enter key and that number was taken by someone else, make sure the other station gets the information right and tell him the new, correct serial number.
This is a practical solution that has been in use for years, also during MS-DOS and CT times. Win-Test does not have a number reserve feature because it (intentionally) lacks a central data master.