Depth header in a lock refreshing method

Under the topic of "Refreshing Locks", it hints that
Client may include a Timeout header. But, Depth header
has not being mentioned.
Under the topic of "Depth and Locking", it discussed
what will happen if "Depth" header is specified for 
creating a new lock. But, nothing was mentioned on 
what's its implication on a lock refreshing command.
Should "bis" document clarify this?
Thx,
-Stanley