Currently to support you in recording non attendance in accordance to the DfE Guidance there are only Subcodes for I and X codes.
Adding further Subcodes for your own recording of attendance can be done using the Add New Record at the bottom of Config>Attendance>Attendance Codes.
When you click Add New Record a blank box appears.
In the first box you need to add a three digit code, this must start with the original attendance code that you are adding a Subcode to. The second box is where your description is added.
Please use the Tick at the end to save the new code.