What's the difference between BK and PGBK black ink?

I believe you have a Canon Pixma like my wife’s. The PGBK is for photographic black and the BK is type print – regular printing.
Both tanks need to be maintained. If you run out of one you will have to replace that cartridge.

@tropical Yes, the mg5200. If I don’t print photos does tge pgbk ink not get used at all?

That is what I understand. My wife got the Pixma for printing of photos for display. It is for use on photo paper for DEEP black.

That doesn’t sound right. My PGBK tank is used must faster than the BK. My BK lasts much longer, though it’s smaller. I never print photos, just text, and always choose “grayscale” so my color inks won’t run out faster. Is that using up my PGBK faster ???