@Ced76 "register" is not a module parameter. It's Ansible task parameter. Take a look at Ansible documentation for registered variables:
Basically what "register: deploy" does is that is says "define a variable called deploy and put a return value of the module in it". A return value of the module in this case are numerous VM facts you can find in "deploy.instance" (sub)variable. Registered variable is accessible in any subsequent task of the same play. Also "deploy" here is just a random name chosen. You can name your registered variables however you like.