HttpLimitReqModule: In the error log, what is the unit of the "excess" value?

[Brian08275660]

Hi,

This is an example of an entry in the log, left by HttpLimitReqModule:

2013/10/27 10:00:11 [error] 1402#0: *313355 limiting requests, excess: 0.580
by zone "zone1", client: 20.147.43.103, server: api.acme.com, request: "GET
/location/locate-ip?key=xxxx&ip=85.210.42.204 HTTP/1.1", host:
"api.acme.com"

What does "excess: 0.580" mean exactly? What is the unit of that value?
Requests? Requests per second? 
There is no information in the documentation, as far as I know.

Thanks in advance,

Brian

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,244205,244205#msg-244205