An important assignment, a mission is focussed.
It works towards the dream and vision you might have, but it is very much grounded in the real world. It is specific. Many missions may achieve a larger dream or a vision.
If my dream is to sell solar power to the world, my first relevant mission might be to become an accredited environmental engineer, or a solar technician.
A mission is an important task and it has a beginning and an end. It is a clarification of what is important and what is not. Do this, don’t do that.
Guided by the vision, a mission zooms in on the day to day stuff that needs to be executed before a dream comes true.
That’s my take.