Hello Saugata,
as per my understanding.
Case 1:
if you have the situation like based on the available quantity you want assign, we can go with alternative group in BOM.
Case 2:
if you want consume as per the your production version.
like component 2 as available and component 3 as per your requirement.
in this Stranded cost will take place for planned consumption and variable cost will captures for the planned & unplanned consumption against the order at the time of order settlement.
Best regards
Venky