How to call a macro ??

How do I call a macro ?
I included a .nsh file (Section.nsh) with macros in it
and tried to call on eof the macros with

UnselectSection firstsection

but the compiler did not accept this command.

Can I put the macro call anywhere in the .nsi script or must it
be AFTER/IN the section I refer to?

In the documentation I did not found explanations.
I expected it as a topic in chapter 5.

It's not so hard, and it actually IS in the documentation. But it is very hard to find 😉. You can find it in Chapter 5 under the header: !insertmacro.

This is an example line:

!insertmacro MUI_LANGUAGE "English" 

Here, MUI_LANGUAGE is the name of the macro, and "English" is a parameter.