From FIXwiki

FIX Field 813

Tag	Name	Type	Enumerated	First introduced	Deprecated
813	ApplQueueDepth	int		FIX.4.4

Current number of application messages that were queued at the time that the message was created by the counterparty.

Notes

Please post comments, clarifications, examples here. Click on the edit button on the right and away you go! 

Don't forget to refer to the FPL Discussion Groups to back up anything you write.
You will often find the authoritative statements on matters of FIX specification interpretation there.
Links from FIXwiki pages to relevant discussions on the FPL site are very valuable.

For example if you were writing a clarification on how to specify the "quantity" of an order in FIX, you might add the
following relevant link, http://www.fixprotocol.org/discuss/read/f5014573, to back up your points.