The activation of Strict mode for timesheets can be found when you are adding a new Timesheets planning or editing an exciting one. Please see the relevant section, to learn more about Timesheets planning.
The Strict mode checkbox is shown at the bottom of the relevant facebox:
When the Strict mode checkbox is selected, users can only create new activities in months where PMs are planned. This means that the month's Timesheet planning cell has a value, even if it is zero.
In the example below, the Lab PI (demo) user will be able to enter activities for January and February, but not for March.
You can learn more about Timesheet planning in the article Step 4 - Adding users planned assignment information (percentages) to the work packages.
Additionally, with Strict mode active, users will receive an email notification reminding them to fill the Timesheet, only in months where they have planned PMs.
If Strict mode is not active, users will receive this email in all months the workpackage is scheduled.