Questions regarding worker_connections

[Alder Netw]

Got a question regarding the worker_connections configuration. Is a
worker_connection
essentially
a tcp socket connection or http session?

Via the http_stub_status_module, I can see there are
connections in waiting state, but any new request
will result in a "worker connections are not enough"
alert in error.log. Why it cannot accept new connections
if there are existing connections in waiting state? Is that
a bug? I am using nginx 1.6.2.

Thanks,

Alder
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