Canon BJC-250 - Manual (Page 65)

3
Press in on the storage box right below the tab.
Then press down to lock the cover in place. Close the
storage box.
Whenever you remove a BJ cartridge from the printer, store the cartridge in the
storage box (unless you are disposing of it).
If you have more than one Ink Cartridge Storage Box, you can hook them
together. Simply connect them by the slides and grooves on the sides of the
boxes.