The "calculation software" and attachment which you provided are not available for the OM USA branch. From the part number, PKP268D28A-L
which is the Japanese version, I can verify that the Holding torque is 2.3 N-m and the Rotor Inertia is 490 x10^-7 [kg-m^2].
For the true definition of the B in Tb, you may wish to contact the branch which offered the attachment. I have provided a link below to the global support network:http://www.orientalmotor.com/company/gl ... pport.html
However, referencing your (semi current) statement from before, I believe that the Tb is referring to the holding torque at motor standstill. This specification would have to take into consideration the driver which is being used with the motor. The recommended driver has an automatic current cutback feature which sets the current to 50% when the motor is at standstill. Therefore, at 50% current, it would make sense that the Torque would be 1/2 of 2.3 [N-m], or 1.15 [N-m]