When a module is marked as optional, the completion of that module is not included in the completion status.  This means that an optional module does not need to be completed for that course to be completed.  For example, if you have two modules and the second is marked as optional, the first module is the only module that will need to be completed.  At least one mandatory module is required in a course to ensure completion can be calculated correctly.

To set a module as optional, navigate to the "Content" sub tab of a course and click on the switch next to the module (see image below).

Learners will see a grey "OPTIONAL" tag next to any optional modules.