MG 2.1 Difference between sitemap options

[mjda]

Can someone tell me the difference between "Media item", "User album index", and "User media index" in the sitemap generation options, please?

I'm assuming media item is each individual image. Then user album index would be the different albums. Then what is user media index?

Also, if I'm wrong about the other two, please correct me.

 

[mjda]

Anyone who can help out with this? I'd really like to know what each of these options means so I know whether or not I want them in the sitemap.