'Substitute Supply' & 'Substitute Demand' Order Type in Replenishment Planning

Summary:

We have enabled "Use of Related Items Supplies" Plan option in Replenishment planning. After running plan, we received 'Substitute Supply' (order type) recommendation from Replenishment Planning for substituting one item with another.

Question: After receiving the substitution recommendation, we see that this line is not 'Releasable' from replenishment plan. How do we signal the execution application to substitute the item?

Is this substitute feature available just for analysis purpose only? can't we release back to execution system so that execution application like OM/Inv understands when to use substitute supply..

Just want to understand mechanism of substitute item feature.