Yerzhan,
What type of Project Stock are they selecting on the Project Header? This has a very large impact on this discussion.
If your customer has selected valuated project stock they must do a GI from inventory to the project for the costs to hit the project, otherwise the costs stay in inventory.
I would not create manual reservations if you cna avoid it because someone will always forget to create them and the MRP calculations will be messed up.
Ken