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Somewhat obscure but so simple!
These are the things that make make me think I'll never completely feel confident with latex: I didn't feel that tikz should parse something inside the math mode as a tikz command... and I don't really understand why it isn't parsed if it's in braces.
Thank you for ...

Hello,
have a strange problem running the following code: this works

\documentclass{standalone}
\usepackage{amsmath}
\usepackage{amssymb} %carica anche amsfonfts
\usepackage{pgfplots}
\pgfplotsset{width=20cm,height=7cm,compat=1.9}
\usetikzlibrary{arrows.meta,angles,quotes,decorations}
\begin ...

Thank you, exactly what I was looking for, I'll have to have a better look into this \NewDocumentCommand!

Hello everyone,
I'm actually an old member, too old... my account was deleted somewhere in my last 10 years of absence.
I recently found my self using Latex again so I think I'll be around more regularly!

To start looking at what's happening you should press the "Code" button and place between the two bracketed commands a piece of working code (from \documentclass to \end{document})
It would also help if you eliminate distracting pieces of code that are not involved in your problem.

I think I found the problem. If no argument is inserted, an empty value is returned which is different from a "NoValue" value, therefore \IfNoValueTF returns always false.

I found a couple of soutions to deal with the problem:

The first does not require packages but uses some somewhat cryptic Tex ...

I think I'm missing something: your reply seems logically what should do the job but doesn't work as I expected, in the No Value option the second option is executed with no value,
I changed a little the code to make it clearer I hope:

\documentclass{article}

\newcommand{\F}[1][]{
\IfNoValueTF ...

I'd like to have a command with optional input that does the following:
if there is no input the output would be A_{ab}
if I input [cd] the output would be A_{ab to cd}
At the moment I use \newcommand like this
\newcommand{\A}[1][]{\ensuremath{A_{ab #1}}}
but in this way I have to input "to cd ...