Image Editor questions

I’m confused by the Image Editor. Not in theory but in practice and mostly with the little menu bar. Here’s what I do understand:

  1. The image icon (Browse Image to be linked) is a set of existing images (existing within the scene)
  2. Image name (self explanatory)
  3. Create new image (allowing one to make a basic image controlling size, base color and whether or not it has an alpha)

What I don’t understand:

  1. What the heck is the “F” button there? “Save this datablock even if it has no users” What? what “users” is it talking about?
  2. I expect the “X” button to be “delete” but instead it’s Unlink datablock. What is that? What is it unlinking it from?

Also, that last scuplt I was working on, I started painting a texture, closed Blender and re-opened and that image was gone. Do I have to save each image every time I make a change? So if I create an image with the plus button and don’t save it externally, will it exist when I re-open that Blender scene?

Thanks for any info!