Some languages expand a single character into multiple characters for sorting. For example in English the ligature 'æ' might be sorted as 'a' followed by 'e'. To represent this in a rule, the reset character (&) is used. The idea is to reset the current sorting key to an already entered value, and create multiple entries for the ligature. For example:
"... < a < b < c < d < e ... < z & a = æ & e = æ ..."
This rule resets the sort key to that of 'a', and then enters 'æ'. Then resets the sort key to that of ' e' and enters 'æ' again. This rule says that 'æ' is exactly equivalent to 'a' followed by 'e'. Alternatively ';' could have been used instead of '='. This would have made "ae" less than "æ" but only by a secondary difference.