the lock mechanism

hi,
 i found this piece of documentation on user locks in the postgresql source code in the file lock.c
"
User locks are handled totally on the application side as
* long term cooperative locks which extend beyond the normal
* transaction boundaries. Their purpose is to indicate to an
* application that someone is `working' on an item. So it is
* possible to put an user lock on a tuple's oid, retrieve the
* tuple, work on it for an hour and then update it and remove
* the lock. While the lock is active other clients can still
* read and write the tuple but they can be aware that it has
* been locked at the application level by someone."*/
could some one enlighten me on the meaning of " locked at application level in this context..!!
thanks
sumaira
 
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