If all elements of a lesson are to be locked, activate 'Lock (X)' for the lesson in question. A locked lesson will also be marked with an asterisk '*' in the timetable. Please note that you cannot remove this lock by clicking on the <Lock period> button.
If you lock a lesson for which all periods have not yet been scheduled, the missing periods will be set at the beginning of optimisation, but then they cannot be moved (switched) in the course of the algorithm. This results in significantly worse optimisation. For this reason, please only use this code for lessons that have been fully scheduled.