One major difference between a capsule and the next stage car seat/child restraint is that the capsule is a mobile child restraint with a handle which can be removed from the vehicle with your baby within it, whereas the next stage child restraint stays permanently in the vehicle and the baby is removed from the child restraint at your destination.
It is practical for a base unit to be used with a capsule as it enables you to easily clip your capsule in and out of the vehicle when you travel. It is convenient to use a base unit with a capsule as it means it's not necessary to install your capsule every time you travel.
However, as your baby grows and becomes heavier, their child restraint also becomes larger, heavier and more cumbersome and it is not practical to be removing them both from the vehicle.
For this reason, the majority of the next stage child restraints are installed permanently in the vehicle and do not need a base unit. They are installed directly onto the vehicle seating using either the vehicle belt or the ISOFIX/LATCH system and are not often removed.
The Avery Capsule Base Unit can be used as a base for any Edwards & Co Capsule, but it is not compatible with any other child restraints (either capsules or next stage child restraints).