DOMAIN_ELSEWHERE_AUTO() is similar to DOMAIN_ELSEWHERE() but instead of a hardcoded list of nameservers, a DnsProvider() is queried.
DOMAIN_ELSEWHERE_AUTO is useful when you control a domain's registrar but the DNS zones are managed by another system. Luckily you have enough access to that other system that you can query it to determine the zone's nameservers.
For example, suppose you own a domain but the DNS servers for it are in Azure. Further suppose that something in Azure maintains the zones (automatic or human). Azure picks the nameservers for the domains automatically, and that list may change occasionally. DOMAIN_ELSEWHERE_AUTO allows you to easily query Azure to determine the domain's delegations so that you do not need to hard-code them in your dnsconfig.js file.