Class Manager ships with an example list of absence reasons for when a student can't make a class. You'll have the opportunity to select these when the student is marked as not present on the class register.

After selecting the reason, it'll show on the register like below:

These reasons can also be used to run reports in the future and understand why someone is attending/achieving as you expect.

On the main navigation panel select Settings > Options:

Under the options, you'll see a widget named Reasons, with a few of the existing entries such as Holiday and Injury.  At the top right of that widget, you'll see an edit button. 

You'll be presented with a list of the current reasons. You can remove all the existing reasons if they are not a good fit for your business and add new ones.

To add another one, hit the Add button and a new line with empty fields will appear. Just populate the fields and ensure the name field is unique - this will be the option on the dropdown you are presented with.

Once you are completed and happy with your new reasons, click to save the settings at the top of the page.