I think it's supposed to be for dealers to use, but like all Shimano stuff, available to the public.
As for the braking action, I used to use it as the only brake most of the time on the Torq to avoid rim wear on the hub motor wheel. It's always best to brake well ahead of time anyway so as not to get caught out by something, so just applying it normally gives the necessary braking without feeling the need to pull too hard and generate a locked wheel.
The action is progressive, so it will generate the stopping force if given the chance, and the only time I used the front brake was for any sudden need for fiercer braking caused by another road user.
When I turned the Torq into the T bike with rear motor and V brake there, the back wheel went to the front and the roller brake then had to go since it isn't suitable as a fast bike front brake, that being where rapid action is needed. In addition, the front brake now does most of the work to avoid rear motor wheel rim wear.
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