Supporting Reference Header Level

I need some help with Supporting References in AHC Accounting Hub. I know what SR with balance and without balance means.. But am confused between header level and line level supporting references. I can only see SR being attached in JERS (journal entry rule set) in lower bottom which basically assigns it to journal line. (credit or debit). But is there any option where SR can be attached to journal header also ? What does header level SR mean ? I also see some sources in Manage Sources and some show header vs line. How is that helpful ? Kindly clarify.