This is where you can configure mutually exclusive groups for the same day.
For example, if you want to ensure that English and Maths for Grade 8a never happens on the same day, you could tag column a for both of these groups.
You are not limited to in any form, you could for example ensure that English does not happen on the same day for two classes as well as the above example. It would simply be setting column b for the two groups in question. ( if column a was not in use, you could have used that column).