I was reading about it but I’m not totally sure how apply on Trellis. In Trellis, how can we define it?
Develop: I develop WP loacally.
Stage: I deploy WP to a subdomain of my own domain for show it to customer, For example:
customer-project.mydomain.net
Production: Working WP at final location at the customer domain. For example:
customer-site.com
Is it? Thank you.
Staging can be used more for testing of the website also, before it goes to production/public live site. For example sometimes you can’t test payment processors, geoip,… locally so it’s easier to have it somewhere live and test there.
The idea is also that you have the live/staging/even local environments always the same (as much possible) so same php versions and settings etc. That way if you do something locally it will probably also work on staging, live site. But to be sure you test in on staging.
But sometimes you can’t really be 100% sure that it will work on live either, so you test it there also (if payments work etc.) it usually does but there is always that “i’m not 100% sure” feeling until you test it 
Better safe then sorry i guess.
Good explanation, thank you!