EDIT: Domains only apply to the current internet and not SAFE.
Subdomain is the same as a domain in almost every fashion. Its the application of it that muddies the water on the internet. www. is a subdomain of the domain following it and web servers allow www. to be mapped to the following domain or the following domain to be mapped to www. or no mapping.
Note the following is handled somewhat differently now with deregulation and commercial pressures.
.com - these domains got the title TLDs (top-level-domains) handled by internet authorities (eg .com .org .au .uk .nz .info and so on)
.e - these domains handled by the registrars associated with the TLD they handled
.d - these domains are what is usually referred to as sub-domains (eg www.) and handled by the “owner” of the .e level domains
.c - another domain sub to .d level handle by the .d domain “owners”
.b - and down another level
.a - last level of domains that the person created in the DNS, but it can go further.
The principle that each level is a domain in its own right and a sub-domain of the level above it.
There was a time in the past where users could have their website as a sub-sub-TLD (ie a sub-domain of the website hosting them). Often done by ISP for their customers.
SAFE is simply condensing out the domain structure into one name. The sub-directories are the same as for any web-site.