What are DOM levels?

Question

I\'ve heard DOM level 2 and DOM level 3 mentioned in previous posts. Are these abstract constructs like the ISO OSI model which has layers 1-7 but are only loosely followed?

Answer 1

Looking for levels of DOM? (MDN reference can be found here, W3 reference also is here)

Alternatively, this could be referencing hierarchy of elements within the DOM. e.g.

Hello, World!

being level 1, level 2.