Parents should avoid buying the boosters on this "not recommended" list. These are seats that don't provide good belt fit — the main job boosters are supposed to do. Some boosters can be used in both highback and backless modes. For those seats, the "not recommended" warning applies to the mode or modes indicated.
If you already have one of the seats on the not recommended list, don't throw it away. Any booster seat is better than none at all. But take a look at the belt fit, and if it's not doing a good job, replace it when you can with one that works better.
BEST BETs are seats that provide good belt fit for typical 4 to 8 year-olds in almost any car, minivan or SUV.
GOOD BETs provide acceptable belt fit in most cars, minivans or SUVs.
Boosters that are not recommended don’t provide good belt fit and should be avoided.
Check fit applies to IIHS-tested booster seats that have varied results depending on child size and vehicle model.