I underlined them in red beneath. Screenshot:
It refers back to the section Forms
The word unless has a rather complex history as a word. In Middle English there are various expressions of lesse, of lasse, *on lesse" "not upon lesse thann" and so on. The OED calls these different spellings of unless "forms" and enumerates them by Greek letters. It describes the origin of each "form". Click on the "... show more" link to see the list of Middle English forms.
In modern English, all these forms have been merged into a single word unless.
This is a particular usage for the OED, not in general use in English.