Eurocontrol CFMU - ATFCM Users Manual (4.4.2. ATC)
d) A slot tolerance (-5’ to +10’) is available to ATC to organise the departure sequence.
When I worked in TWR, we were constantly reminded by our regional Flow office that if there was no departure sequence we had to stick to the CTOT, the slot window was not valid (a note allways posted on the GMC position and in every fax they sent about acft having departed not complying).