An all in two nappy is essentially a PUL shell (cover) with separate inserts that normally popper into place in the cover. The idea being that as long as your baby has only wet the nappy you can simply swap out the insert when changing (and give the cover a quick wipe too if you wish). Although if your baby has dirtied the nappy you will need to change the whole thing. As inserts are relatively cheap and covers more expensive this becomes a more economical option because you can use a cover for around 4 times before changing. Also covers are very quick to dry, whereas inserts take a lot longer so you will definitely need far less covers.
I personally don’t find this system suited to younger babies though as they tend to have dirty nappies at every change, but once this stage calms down they are really useful.
A lot of Ai2 nappies feature stay-dry liner as well. For me these are not true Ai2 nappies, I call them hybrid Ai2s as they are somewhere between an Ai2 and a pocket nappy.
These hybrids normally have a pocket opening at the back of the nappy so you can add additional inserts. And the poppers on the inside so you can secure a snap-in insert. If you keep the inserts snapped-in (so between the stay-dry liner and your baby) you can still just swap out a wet insert at change time, although you won’t be able to wipe down the cover. However if you have put the inserts into the pocket instead then you will need to change the whole time at change time, just like a normal pocket nappy.
Here are the Ai2 nappies I have reviewed: