Shopify Source ID helps identify the Web site or POS register that entered each order. If you have one web site, no problem, but you may have many POS registers or apps.
Each iPhone, iPad, or Android device you put Shopify POS on is tracked as a Register. If you accept Cash, Shopify POS will keep track how much you should have for each.
In turn, BigBusiness can use the Source ID to identify the Register and set any presets. For starters, you might want Entered By filled in on each Invoice for future reference.