This change will take effect as of the next new timesheet the user accesses. Once the rule is in effect, the user won’t be able to submit timesheets without first selecting an activity for each entry.
1) In configurable timesheets:
This option applies to projects as well, so if the employee has access to projects, they can select a project rather than an activity to satisfy the validation rule.
Timesheet submission will only be prevented if you have the Errors prevent submission or Warnings prevent submissionworkflow condition enabled in the timesheet template, and the Warning (ON) / Error (OFF) switch is set appropriately in the Activity/Project Required rule (e.g. it's set to Error if you have the Errors prevent submission condition enabled).