Milter-greylist

How is SPF processed in absence of "spf" rules or "nospf" keyword
in config? The manual is not quite clear on this default processing.
http://hcpnet.free.fr/milter-greylist/greylist.conf.5.txt

Empirically I see that milter-greylist does by default quickly pass
those source addresses which fit their domain's published SPF rules,
but it does not for example block senders which are not in the range
defined by the MAIL FROM domain's SPF record (it does log "SPF return
code 4", and BTW a list of error codes translated into human-readable
text would be a nice addition to the docs, or textual output - to the
code).

So... what is the definition of the expected default behaviour? :)

Thanks,
//Jim