Ink cartridges. Instead of paying up front, they send you cartridges for ‘free’ and automatically dispatch replacements whenever they get low (or if you have a problem like they get so gummed up the head clean can’t fix them) If you don’t want the subscription, you have to buy your own carts retail, then it’s just like a ‘traditional’ printer.
The sub itself isn’t (in my opinion) scummy just a value proposition. What is a bit scummy is that the carts that come with the printer are yours, but you can enroll them in the sub service. You can do that the day you buy it or right before they run out. In theory they’re supposed to revert to ‘owned’ if the sub is cancelled, but I’ve heard there can be hoops to jump through. The carts that come with the printer are tiny compared to either retail or rental so won’t last long either way.