Partial class

Partial methods are implicitly private (the default for methods with no access modifier) and can only be called from within the same class.

It would be problematic for them to be called from other classes because, if the partial method is not implemented, it will be removed altogether by the compiler and this could have repercussions on other code. Consequently, access modifiers are unnecessary and are not allowed.

you cant modify it because you divide the main class if your class will contain many things you divide it.