From HEXONET Wiki
"Web apps" are services which allow their customers to use their own domain, that is to associate their domain with their web app account. Many online services, such as blogging and email platforms, offer their users the option to use their own domain
A web app is a service with which you can use To connect a web app to a domain
You can use any of them to have all respective records of the reference zone copied over to your zone. Any changes to the respective records of the reference zone will be constantly monitored and your zone will be automatically updated accordingly if there are any changes. No additional lookups are made when your zone is queried so this is the high performance solution if you want to manage only one dedicated authoritative DNS zone but utilize the respective A, AAAA, MX or TXT records for as many other DNS zones as needed.
PLEASE NOTE: If you set a X-ALIAS-<RR> xDNS record for your zone then all previously existing resource records of the same type and will be removed and then the respective RRs from the reference dns zone will be set.
X-ALIAS-<RR> Records have the following syntax:
yourdomain.com. <TTL> IN X-ALIAS-<RR> <REFERENCE-DNS-ZONE>
PLEASE NOTE: The secluding dot after the domain name (www.yourdomain.com.) is required in any case!
yourdomain.com. 3600 IN X-ALIAS-A zenfolio.webapp.ispapi.net.