Apologies, I don’t make sense to myself either!
What I have (on the backend) is a ACF Taxonomy field call “selecttaxonomy” which is a dropdown of EDD categories and I can pass this into a php that lists what products are under that category but this means changing the backend every time as you can’t get the taxonomy field to be a choosable field (dropdown, checkbox, anything) on the front end.
I then created a seperate dropdown and have tried to pass the choice into the original php code but can’t.
As for the <form> command, I got the code from the Codex & haven’t got a clue what it does!
I hope thia makes more sense this time.
