If you are performing a check with RLT, then the system assumes that the material is always available at the end of the replenishment lead time. Hence, even though it is a BOM material, it will still be considered available at the end of the lead time.
Talk to the end users and ask them if the RLT should be considered when performing ATP.
Based on the above information, there will never be a time when a line item will not be confirmed in the sales order. Even if it is ahead of the requested delivery date, every line item will be confirmed.