Apexification is the treatment of choice for a permanent tooth with wide open apex when:
Apexification is the process of inducing the development of the root and the apical closure in an immature pulpless tooth with an open apex.
The main aim of apexification is to preserve the Hertwig’s root sheath and apical pulp tissue.

Ref :Textbook of endodontics, Nisha Garg and Amit Garg, 3rd edition, pg no:548
The pulp is necrotic
The pulp is vital
The pulp and root canals are calcified
There is traumatic pulp exposure during cavity preparation