Web Alias Domains are a unique SlickDNS feature that allow you specify a list of domain names that should be considered as aliases of a master, primary domain. You don’t need to do any configuration of the alias domains as the root/apex records of the primary domain are automatically cloned for the alias domains. (Optionally you can redirect an alias domain to a URL.)
When is this useful? With the explosion of top-level domains it’s common to buy a set of similar domain names to protect your brand even if you intend to use just one of the names on your public website. Or you might buy special purpose promotional domains such as example.jobs which would redirect to http://www.example.com/jobs/.
Suppose you’ve just bought example.com, example.net and example.org and you need to do the DNS configuration but you just want example.net and example.org to use the same address records as example.com. With the SlickDNS domain editor you can just configure example.com once as a regular domain and then add example.net and example.org as aliases and they will automatically pick up the configuration from example.com. (This exact scenario is created for you as an example configuration when you create your SlickDNS account.)