These are the requirments needed in 72 hour mode to unlock different endings. The farther down the alphabet an ending is, the worst ending you have (i.e., 'F' would be the worst and 'A' would be the best).

Ending A ... Solve all cases. Talk to Isabella at 10 AM and return to the heliport at 12 noon on Sept. 22nd.

Ending B ... Don't solve all cases but be at the heliport on time.

Ending C ... Solve all cases but do not talk to Isabella at 10 AM on the last day.

Ending D ... Remain a prisoner of the Special Forces by 12 noon on Sept. 22nd.

Ending E ... Don't be at the heliport and don't solve all cases.

Ending F ... Do not collect all of Carlito's bombs during case 7-2. You do have to successfully complete all the cases up to this point.