In general terms, it is often not permitted to create an ALIAS or CNAME for your domain's apex (or root domain).
If your domain name is example.com, then example.com is your "apex" or "root domain" (while www.example.com would be a "subdomain" or "subhost").
Usually it goes against "The DNS Rules" to create your apex as a a CNAME or ALIAS, because it breaks a rule within DNS that a CNAME record cannot have any other records within the zone by the same name. (So the presence of your SOA record and your MX records would break this rule, and in time, your zone)
Somehow, ZoneEdit implemented a workaround to this which allowed literal aliases for the zone apex. (We think we know how they did this, but it doesn't matter, because....)
On the new system, this is implemented in way called "ANAME". Which you define in the same way as a CNAME, but under the hood this gets translated into an A Record or "hostname" (In fact in the user interface you just enter this as a regular CNAME, the system does the rest).
The big advantage to this is it enables you to also define MX handlers for your domain names using ANAMEs - thus enabling you to receive email at your own domain names, while under the old system, you couldn't.