What does the [Userlimit:warn] in the Apache error log mean?

In the log see more like the following error:

[Sun 25 10:35:03 2013] [10970692] [Userlimit:warn] [client 119.147.225.xxx:58664] Vhost limit:access to/xxx/xxx/xxx.gif deferred, requested Ho St xxx.com, Username 10970692 Userlimit (exceeded currently), Referer http://xxx.com/index.php

I don't know what the limit is,
There will be a few consecutive times,
There will be twenty or thirty of these records in a row,
The continuous record IP is the same, and the time is between two or three seconds.

The Apache plate doesn't seem to be active here, so it's written here ...

Reply to discussion (solution)

The request is too frequent, exceeding the limit of the virtual host.

The request is too frequent, exceeding the limit of the virtual host.

Then what is the instruction set? I can not find.

Maximum number of concurrent users configured in Apache

Maximum number of concurrent users configured in Apache

The original in the MPM command inside, thank you moderator adult ...

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