TW,
Let me explain in a different way.
1. The material is available in ample quantities, but due to different lead times like pick/pack, transportation lead time, transit time etc., the delivery couldn't be confirmed for the reqeusted delivery date. Eg: Order created today with RDD as tomorrow. But, the other lead times are 3 days. In this case, the delivery cannot be confirmed for the RDD.
Will the OP consider this as non-availability of material?
2. The RDD is in another 4-5 days. Currently the ATP quantity is zero. But, the material can be produced internally in 1 or 2 days (lead times) and the delivery can be confirmed for the RDD.
Though stock is not available at the moment, how the OP considers this situation. Should the system raise a PR initiating the third party process?