What is the selection criteria of D in HSMM?

Hello,
Thank you for your open code!
What is the selection criteria for setting value of `D`? Why set `D` to `maximum segment length` in HSMM ?
If `D` is set as the maximum segment length, then the state corresponding to some smaller segment must also last for `D` time to jump to the next state? Compared with HMM, will the result be worse?
Could you help me?
Best wishes