Help with Advanced options

Hello there!
As it is no manual regarding RTA to see into it, I would need some help regarding the Advanced options.

There are three options available, and I would like to use all of them actually. Of course, I will do a backup first of the database and files before doing nothing.

1) Remove metadata: I understand they will remove info about images used in old plugins present in the wp_post_meta table, right? Is it sure to use it?

2) Remove thumbnails: Just to be sure that I have understood how it works. I must select the checkboxes at right of all the sizes that I want to be removed, right?

3) Remove images that does not exist: I understand it will remove the references in wp_post_meta table of images that are not anymore in the hard disk, or which name have been changed, right? Is it sure to use it?

When using shortpixel I have received lot of errors, because it could not find the images to regenerate or compress them. So if I’m not wrong, to delete all these references in tables would be awesome in my case.

I hope you can help me in these questions.

Thanks in advance!