Any part that keeps the aircraft from being released to service is considered AOG.
Since an aircraft sitting on the ground unserviceable does not generate money, the term AOG justifies higher purchase costs etc.
For normal on the shelf spares the company has the luxury to shop around.
When an aircraft goes AOG it is costing them money thus they will be prepared to go to greater lentgh to obtain the required part.
That is where loanparts come into view, companies will loan each other parts at high fares.
Availlabilty costs can start at 30% of the value of the part, even if only used for one flight!