When you create a subject, the default settings for the subject will require completion. If you choose to turn off this setting for your subject, it will be labeled as reference material and will not track completion percentages.
To enable/disable "Require Completion of Subject?"
Navigate to the subject you want to edit
Click the three dots to the right of the subject title
Click "Advanced Settings"
Scroll down to the bottom half of the modal
Toggle on or off the "Require completion of Subject?" button
If you turn off require completion for the subject, it will be displayed as "Reference" to you and anyone assigned. You'll notice when you toggle off require completion, the rest of the settings are hidden (as shown in the video above).
Reference Subjects will have a "Reference" tag on your Subjects page as shown below in both Admin and My Desk view. This helps you know which Subjects are reference only, and which are not.