Use of symbols '@', '&', '=' and '>' in custom directive's scope binding: AngularJS

Question

I have read a lot about the use of these symbols in the implementation of custom directives in AngularJS but the concept is still not clear to me.

What does it mean if I

Answer 1

I had trouble binding a value with any of the symbols in AngularJS 1.6. I did not get any value at all, only undefined, even though I did it the exact same way as other bindings in the same file that did work.

Problem was: my variable name had an underscore.

This fails:

bindings: { import_nr: '='}

This works:

bindings: { importnr: '='}

(Not completely related to the original question, but that was one of the top search results when I looked, so hopefully this helps someone with the same problem.)