What is this bath stain and how can it be cleaned?

[elsewhereelsie1]

Any ideas? It’s been like this since I moved in!

Ooo that's really annoying. Does it feel different from rest of bath? Could it be limescale from a drippy shower?

It looks a bit like the bath surface has been damaged - does it feel rough and ‘dry’ in comparison to the rest of the bath? If so you might be best to call in a professional - look for bath resurfacing / repair/ renovation in your area.
Had this in a new shower when one of the tradesmen dropped something chemical on it. You can’t see the mark now

There are home enamel repair kits that I've used with some success, however that looks like too big an area to cover. Does it sit on top or is it etched in?