Wiring diagram in MKTED210011EN catalog shows what looks to be a resister in line with the series winding of poles for DC5 circuits. Is this representing a resistor and how is this value calculated?
There is no resistor. The rectangle you see in the power wiring diagram is the load ( motor). The DC load could also be non-motor.
This represents that the motor(load) needs to have a contact on each side so there is no voltage present when the contactor is off, and that the air gap is additive no matter which side of the motor the contact is on.
Note: The TeSys D contactors are rated for DC applications but are not UL listed / approved for DC applications.