Need some explanation about stepgen

Hello, everyone, I've been reading about stepgen module, I need to understand how it works, I have atached an image and I would be so grateful if someone would help me to understand how the block diagram works, specially the part beteween red lines

You probably don't need ti understand the block diagram to make it work for you.

Fundamtentally, every 1mS you give the stepgen a new position command and it makes as many pulses as necessary to get there in the allotted time.

(You can use it other ways, if you want, but that is the way it normally works)