The two threaded holes inside of the rectangular depressions on the side of the ACC are for attaching accessories. These are spaced using M-LOK spacing which will work with a lot of different M-LOK accessories currently on the market.
Check these accessories below!
Data Card - https://mdttac.com/m-lok-data-card-holder.html
Spare round holder - https://mdttac.com/mdt-m-lok-spare-round-holder.html