All i can find on this is a fairly unclear thread on this site at Pay later online? - nothing in the documentation at all.
1 Answer
"Default invoice payment page" setting is used to set contribution page that can be used to pay pending or partially paid contribution using payment processor.
More about the setting and uses:
Wiki: https://wiki.civicrm.org/confluence/pages/viewpage.action?pageId=346750980
CRM Issue: https://issues.civicrm.org/jira/browse/CRM-19263
HTH
Pradeep