Hello Sally,
In MDVP if you select the Option Check Mode On
Then bottom of screen you have an option to Rest the Availability
Refer SAP help document on it
Check Mode for Collective Availability Check - Procurement Proposal (PP-MRP-PP) - SAP Library
When call the collective availability check. After the list is displayed with the planned orders, you can select planned orders and then start the availability check. The list is updated accordingly, in other words the available quantities are set.
Also note If necessary, change the dates of the planned orders for which components are not available until a later date.
You can check the current ATP availability quantity on CO09
For more details please refer SAP consulting note 423538 - MDVP: Missing part list in collective availability check
Best Regards,
R.Brahmankar