Relevance modifiers change the importance of fields, based on keyword matches.
Relevance for the regular engine
Exact match relevance is calculated without examining each word in the phrase, only checking the exact phrase for occurences. Non-exact match relevance is the opposite, where each word is checked separately.
For most cases the default configuration is perfectly fine. Please check the table below for different cases.
If more than one field is much more important, then make sure to set them both to higher values.