Full Idea
Natural deduction takes the notion of proof from assumptions as a basic notion, ...so it will use rules for use in proofs from assumptions, and axioms (as traditionally understood) will have no role to play.
Gist of Idea
Natural deduction takes proof from assumptions (with its rules) as basic, and axioms play no part
Source
David Bostock (Intermediate Logic [1997], 6.1)
Book Reference
Bostock,David: 'Intermediate Logic' [OUP 1997], p.240
A Reaction
The main rules are those for introduction and elimination of truth functors.