A SLD or “Second Level Domain” is all often called the “domain” or “base domain.”
Most of the time when people buy a domain, they are buying a Second Level Domain. To get even more specific, they are buying a SLD + TLD combination.
When a ccSLD – or “country code Second Level Domain” – is used, then a user will actually be buying a third level domain.
ccSLDs can make it difficult for developers to deal with domains, in the sense that many developers have to parse out URLs for many reasons, but there is no hard set rule on how to parse a domain out. You just have to have a comprehensive list of all ccSLDs and their corresponding ccTLDs in order to properly distinguish the users “base domain” from their “domain extension.”