I don't understand what does BX_ROOT mean in bitrix?

Question

1C-Bitrix

Coders use this line, I don’t understand what it is, the documentation about constants doesn’t say anything specifically about this one, where it came from it’s not clear ...
Some kind of root of something
(I just started to study)
here is an example

if(file_exists($_SERVER["DOCUMENT_ROOT"].BX_ROOT."/.config.php"))

Answer 1

the name of the directory where the Bitrix core is located, /bitrix by default