There are 2 main ways to capture systems on boxes.
If your priority is to capture and see which systems are used, add systems as role on a whatbox.
This is quick and easy, as you are capturing roles you can add any number of systems.
Above, I have added an icon in front of SharePoint so that it is more visual. The system will appear in the role manager, next to all other roles, and can be analysed in the same way as role.
If your priority is analytics add systems as custom field.
Add one (or more) custom fields to a box and capture the systems. A benefit is to be able to capture additional notes / comments in the custom field. This can help for further analysis down the line. Analytics capability with custom field are more comprehensive.
You can decide your own naming convention for the Name and Grouping fields to be more relevant for your project. Example:
If you compare the 2 examples above, the reports will be grouped differently. Choose what is best for your use case!