[Vortex] is it typo?

[dwnam]

hi.

"Vortex Library Manual (C API)" section 1.2
contains the following

Create the listener using vortex_listener_new, specifying the hostname
to be used to listen incoming connection and the port. If hostname used
is 0.0.0.0 all hostname found will be used. If 0 is used as port, an
automatic port assigned by the SO will be used.

is "SO" typo?
does it mean OS(Operating system)?
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