iP4300 - who decides which black to use?

I have a Canon iP4300 - works great. I refill the tanks regularly
myself.

I've always been curious about the two black tanks. Yes, I understand
that one is for dye-based black, and one is for pigment-based black,
and I can look up why one is better than the other for some things.
But what I don't understand is how the printer knows which one to use.

I do mostly "gray scale" text printing, and the large (pigment-based)
black tank is the one that more often needs refilling. (Now, some say
that dye-based inks are better for text, which seems to suggest that
the printer is using the wrong one.)

I would have to assume that the driver settings dictate how the
printer decides which ink to use. I think I saw one post in which the
author said that the "media type" specified by the user in the print
driver determined this, in that the dye based ink was used for "photo
paper" and that the pigmented ink was used for "plain paper". But I
*never* choose "photo paper" in the driver on my printer, and I do
seem to use some dye-based black. And there are many more media
options than "plain paper" and "photo paper".

So there must be a "rule" that the printer uses to chose which black
to use. What is it?

Doug