It’s all a little bit of a mixture, you can use CLI-8’s instead of BCI-6’s but not the other way round.
The same goes for CLI-520/521 they can be used instead of the CLI-526/525 but with these ones you can use either, not that you’d want too.
Then there is the PGI-72’s they can be used instead of the PGI-9’s but the jury’s out as to whether they can be used the other way round also.
Last but not least are the PGI-42’s they can be used instead of the CLI-8’s or more so the other way round, because the CLI-8’s are more common and the CLI-42’s are as scarce as hens teeth..
All the above will need a chip swap to get them to work except the BCI-6’s