Conditions under which a carrier appear in the list of carriers in FO checkout

[arouiadib]

Guys I have some findings and I am not sure if they are already in the specs.

For a carrier to appear in FO checkout > Shipping, conditions are:

• Carrier is active.
• AND carrier is activated in region where customer address belong( if a customer address is in Europe, then Europe should be checked in "Shipping Locations and costs" in BO> carrier Edit page)
• AND carrier is selected in "Available carriers" for the product in BO> Product Edit page > Shipping.

Please correct me if I am wrong. Any ideas I miss are welcome.
It will be also invaluable if someone points me where to put them, so that I create a PR.