Under some circumstances you may want to set up a service as if it had been purchased via 20i on your WHMCS install. This may include cases where you've migrated services from another WHMCS provider.
Autodetect only works when you have a package with a name which exactly matches the domain name in WHMCS. The behaviour is undefined if you delete and recreate a package, or move a domain name between packages.
Importing Stack Users
If you've set up Stack Users, you may want to copy them over so that your customer can access all their services from within WHMCS without having to go back to your StackCP site. This is only applicable to existing unlinked Stack Users, as new Stack Users are set up automatically whenever a new order is placed in WHMCS.
If you use autodetect, you should be aware that this may connect a Stack user which you didn't intend to allow access to. Autodetect only works when the WHMCS user email address exactly matches one of your stack user email addresses.