Petzl offers two Additional Brake kits, the D020DA00 Open Additional Brake kit and the D020EA00 Closed Additional Brake kit.

Version A Version B
Open Brake
D020DA00
Closed Brake
D020EA00

Either kit can be added to a latest-version small (Model S) or large (Model L) I'D. Both kits have a V-shaped notch to provide additional friction.

The open brake allows the user to decide whether to use an equipped I'D as a standard I'D or employ the brake when additional braking is desirable, depending on the situation. The closed brake requires pre-rigging and reduces flexibility compared to the open brake, taking the choice away from a skilled user. Apparently it provides lawyer-mandated enhanced security for untrained or incompetent users - a scary thought, in my opinion.

The kits are intended for rescue lowering applications. I'll leave evaluation for that purpose to the INS crowd.

If the decision between the two is to to made by competent users, I would consider the open brake; but if it is to be made by the company lawyers, then the closed brake would probably get the nod. I do not see a need for either kit for normal rappelling applications, so my choice, for my purposes, is to leave it off.