change the tooltip for pipe priorities to
"Items only move from lower to equal or higher priorities. Greater priority differences take precedence."
or if its fits in the tooltip the longer version:
"Items only move from lower extract (OUT) priorities
to equal or higher insert (IN) priorities.
Greater priority differences take precedence."

gray out inapplicable priorities
insert only (IN) -> gray out extract priority & replace the tooltip with "inapplicable for insert only (IN)"
extract only (OUT) -> gray out insert priority & replace the tooltip with tooltip "inapplicable for extract only (OUT)"