Only one person can access a Fulfillment Request at a time. When a person has a fulfillment request open, all others are prevented from making changes. The request is considered "locked." The request can still be viewed by others when it is locked, but they cannot modify it.
Note:
In this situation, the System is treated like a person, that is, if the system is working with a fulfillment request, it is locked to any other changes. Conversely, if a person has a fulfillment request open, the system cannot perform any actions on it.
If you have the appropriate permissions, you can release the lock on a fulfillment request. (More)