In root end preparation:
Car and Bentkover defined an ideal root-end preparation as "A class I preparation at least 3.0 mm into root dentine with walls parallel to a coincident with the anatomic outline of the pulp space".
Five requirements suggested for a root end preparation to fulfill:

The apical 3 mm of root canal must be freshly cleaned and shaped.
Preparation must be parallel to the anatomic outline of the pulp cavity.
Creation of an adequate retention form.
Removal of all isthmus tissue if present.
Remaining dentin walls should not be weakened.

Textbook of Endodontics Nisha Garg 3rd Ed
Tunnel type cavity is prepared
Class I cavity is prepared
Vertical slot is prepared
Any of the above