Whenever we place the {-wI'} on a verb, we produce the meaning "one who does/is, thing which does/is".

So, if we write {vumwI'} then we have the meaning of "one who works".

But if we wrote instead {vumlu'wI'}, then we would seemingly/apparently get the meaning "someone (unspecified) who works".

So, the conclusion here is that "the {-wI'} used on its own, talks of someone specified, while the use of it in conjunction with {-lu'} talks of someone unspecified" ?

-wI' doesn't mean someone specified does something; it turns the verb into its own subject. A vumwI' is someone who does vum. But a verb with -lu' has no subject, so there is nothing for -wI' to turn the verb into.

-- 
SuStel
http://trimboli.name