A "Committed" Quantity is what is 'in flux'... a result of some transaction(s) that have been entered into a Register (POS, Inventory-Requisition, Inventory-Adjust, etc.)... but have not yet been "posted".
The "effective" Quantity-Available = (System's) Quantity-On-Hand - Quantity-Committed.
Committed's get affected immediately by a Transaction's 'entry'. Committed-Quantities get flushed-over to Quantity-On-Hand upon Register Posting.
Quantity-Available (at any moment in time) is not a system-stored value, but rather is "always" dynamically calculated to be Quantity-On-Hand minus Quantity-Committed.
Example -
Start-of-Day:
Quantity-On-Hand = 12
Quantity-Committed = 0
Quantity-Available = 12 - 0 = 12
During the Day, POS has Sold a Quantity of 3 (and the pending Quantity-effect gets "noted"):
Quantity-On-Hand = 12 (doesn't change during the day with respect to on-going POS).
Quantity-Committed = 3
Quantity-Available = 9 (at that moment)
End-of-Day Register (the Quantity-effect gets "solidified"):
Quantity-On-Hand = 12 - 3 = 9
Quantity-Committed = 3 - 3 = 0
Quantity-Available = 9 - 0 = 9
Note: At the End of the Day, all things being efficiently done (!!!) all Committed's should be zero... meaning everyone in all Application areas properly did their respective Postings. Of course, due to many reasons, this may not be properly done... and Abacus 21 has a maintenance program to "re-set" the Committed (to what was hanging out in the various un-posted Registers etc.).
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