Hi @NataliaYunusov,
Thank you for your response! This definitely clarifies it. 
I do see the reason to have the UI display the lifecycle states in the order determined by the customer, where the initial order for the out of the box lifecycle states can also be updated to the customers needs (or even delete those lifecycle states if other custom lifecycle states are used).
So the fact that the UI uses this field to display them in this order was what I was looking for. This fact is not documented yet. It would be nice if this can be documented in the below location, with the mention that the order as seen in the UI can not be changed through the UI, but would depend on using the API, with a link to the API documentation:
Setting Up Lifecycle States - SailPoint Identity Services.
Personally, I would have chosen a different attribute name (like orderIndex?) since priority would now suggest that the lifecycle states have an order of “importance”, while this is not the case here. I hope this will not confuse customers.
One last thing is that I tested this and I found a bug regarding this functionality. The attribute priority is a number, yet sorting by it will not sort it as if it were a number, but as if it was a string. As a consequence, so the UI will now show this wrong order of priority:
10,110,120,20,30,40,50,60,70,80,90 instead of the expected 10,20,30,40,50,60,70,80,90,100,110.
Can you please fix this @NataliaYunusov?
Thanks again for the clarification!
Kind regards,
Angelo