This is an internal oring in the female portion of the quick disconnect. This is the MOST COMMON cause of leaks in remote kits besides your tank oring. It sits inside a groove down inside the female portion. It's always nice to have an extra one or two of these around. They occasionally blow out if you disconnect your line while it's pressurized. They're also prone to leaking from dirt contamination and a leaking QD oring can ruin your whole day.