It's just that the lapbelt is a manual (pull the tail to tighten, tip to loosen, like an airplane belt) lapbelt, so it has to be set for the person sitting there... an adult has no trouble tugging the tail to tighten and loosen it, but a kid has a hard time, especially working around a booster armrest), and then the shoulderbelt hooks into that, so it's just more unwieldy next to booster armrests in general. The manual doesn't prohibit booster use, and it's not an unsafe type of seatbelt by any means, but it takes a lot more proactive work on mom's part to make sure the belts are all tightened and buckled properly. (My 11 yo dd used to go into a fit of tears when confronted with that belt, so we had a few lessons on how it works and now she's cool with it, finally!).