difference between setq and setq-local

[Pankaj Jangid]

There are three variants of setq; namely - setq, setq-local,
setq-default. Meaning of setq-local and setq-default is clear from the
docstring. i.e. changes made by setq-local are not visible outside the
current buffer. And changes made by setq-default are visible in all the
buffers in which the value is not set (overridden).

What about plain setq? My assumption is that these changes are visible
in all the buffers if not overridden by setq-local. In this case setq
acts like setq-default. Or is it the opposite? i.e. setq overrides
setq-default valuep; in which case it acts like setq-local.

What am I missing? In which scenarios the three functions behave
differently from each other?