The "top" refers to the first level.
Top level: COM
Second level (where domain names are registered): example
gives example.COM
Third level example where domain names are registered at the third level:
Top level: UK
Second level: CO
gives example.CO.UK
There are some ccTLDs (country code TLDs) such as .US that have even more complex structures that allow registration at a fourth and fifth level.
https://en.wikipedia.org/wiki/.us
The ccTLDs generally follow the ISO3166 two letter standard for country/territory names.
https://en.wikipedia.org/wiki/List_of_ISO_3166_country_codes
The second levels/sub-domains such as .co/.com, .net, .org were generally a pre-search engine and pre-Web ideas to categorise registrations by use (commercial, network, non-profit organisations, government, military, academic). As new ccTLDs were added, they often used the com/net/org/edu/mil/gov as second level or followed the UK (co/net/org/ac/gov) Spanish (com/net/org/gob/mil). Some ccTLDs do not allow registrations at the second level and the .UK only introduced registrations at the second level (example.uk) a few years ago as did the Spanish .ES ccTLD.
This is the list of legacy gTLDs, ccTLDs and new gTLDs:
https://www.iana.org/domains/root/db
The "new gTLDs" generally refers to the 2012 round of gTLDs that were added. That was about 1,200 extra gTLDs. Some of them are struggling to get a few hundred new registrations a month. There were also .BRAND gTLDs where companies could have their own private gTLD (.google for example). Most of the new gTLDs that have been terminated (ICANN-speak for deleted) have been .BRANDS. Unfortunately, the 2012 round was a solution for a problem that had been solved by 2009 and the demand for most of the new gTLDs had disappeared by the time the first of them launched in 2013. (Covered in the free pages of the Domnomics book).
Regards...jmcc