If you go to their website :
www.niceic.com
and look under the heading "contractor", the 2nd option is "niceic schemes".
Click there, and they`ll give you the differences - note that DI has 2 distinct levels -A & B.
If you`re only going to do domestic, and don`t envisage doing any council or large housing assoc. work, DI should be fine.
Some, though not all councils and HAs will only accept work from ACs, even though a DI is technically capable of the work. There has been prior discussion of this, the upshot being that, as customers, they can ask for whatever standards they want!
HTH
KME
n.b. If you register as an AC, and do domestic works, you MUST also register with them for DI; otherwise you don`t get into the building control notification site.
KME (Approved Contractor).