Ssxx feature

Could you describe me how the stylistic feature works? I created with the + button a new feature and named it ss01. In the lower field on the right side I wrote:
sub Q by Q_Alt;
I tried the button “Compile” and also the box “generate Feature automatically” but neither worked. Could you help me?
I posted a screenshot from the error message.

  1. “Q_Alt” is not a good glyphname. Make that “Q.ss01”. Before the dot, you should have a valid glyphname that can be broken down to one or more actual characters. Use the underscore only for ligatures, e.g. “f_f_b”. After the dot, it’s best to write the name of the associated OT feature.

  2. If you stick to that convention, then automatic feature generation will work. Just press the curled arrow, and ss01 will be created automatically. No need to press the plus button.

it works, many many thanks!