tv_grab_combiner: what does it do?

All,

Having read the tv_grab_combiner manpage and having tried it on a couple 
of grabbers I still wonder what it does and what its intended use case is.

That is: is it a way of running multiple grabbers followed by a tv_cat 
all in one go, or does it also try to correlate data for a particular 
channels from different grabbers?

FWIW: I'm looking for the latter, with a view to making my system more 
robust to glitches in the metadata supply.

Both of these are a form of "combination", and the manpage does not say 
which it is. There is also no wiki page.

Thanks, Jan

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