What is the syntax? (prefix - underscore)

Question

In many programming languages I meet: the prefix of variables or functions - the underscore. What is this for?

For example, here is an excerpt from the vk script:

if (vk.al != 3 || vk.navPrefix != '!') { ___to = ___htest.replace(/^(\/|!)/, '');

Or

 int ____MAX, ...

Why so much? I understand when one set, sometimes conveniently in such cases:

 int x = ...; int _x = ...;

Well, for example, in Python, underscore characters are added to overload operators to indicate the importance of the method.
For example, init is a class constructor, add is an overload of binary multiplication of class objects.
In short, the more underscores, the less the author wants to ever touch it.
There is no convention, but languages are often spurred by prohibitions (PHP) and sentences (python) so to do (in fact, only the documentation forbids puffs).
In many languages, the system functions, variables, and constants are designated in this way.
Also, often, underscores are used in OOP to denote private methods and class fields, for example: class SomeClass {private: int _x;
In internal regulations it is quite convenient to introduce different notation for syntax, no one forbids it.
It is sometimes more convenient to work with such a code if you don’t overdo it))

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