The "d" or "m" signifies an internal routing, which means the call will be routed to another service within your account. This may be required to accommodate your custom routing requirements and is often used in conjunction with the time of day routing feature and the IVR feature.
More commonly, you will find an "m" rather than a "d", but both perform the same function. However, while two services linked with d-termination will create two call records, the m-termination will merge the two call records into one. This will ensure that your call volume is not unnecessarily inflated.
If you notice that a service uses the m-termination, find the corresponding linked service to see how your calls are routed.