Before starting to inventory a set which requires effort and time, the submitter can reserve the set to let others know that they are doing the inventory so that others don't inventory the same item resulting in double work. If the reserved item is not submitted for approval within 3 days of reserving the item, the item will be unreserved. An inventory cannot be submitted for an item which someone else has reserved.
An item can be reserved to be inventoried on the Add Inventory page.
You can also view all currently Reserved Items accessible from the Catalog Inventories main page. "Inventory in Progress" shows on the catalog detail page for items which are currently reserved to be inventoried.