Essential function is incomprehensible

I’m ashamed of this plugin and this company, the only thing they do is send push messages to devices. Would it cost to leave the userId or ExternalUserID part minimally better developed?
The way it is done is disgusting and the documentation is shamefully poorly written, on Google or YouTube there is nothing explaining this part better and only extremely advanced users will understand it. I’m embarrassed to go into this plugin’s documentation and see that there wasn’t the slightest effort to make something a little better written.