You're confusing different tags.
There are docker image tags, which are found on the docker hub page under tags. If you don't select a tag, "latest" is selected by default, which is what you're seeing for our image there, simply because you put in "linuxserver/plex" as your image in the container settings. If you change that to "linuxserver/plex:1.15.1.791-8bec0f76c-ls11", then you'll see "1.15.1.791-8bec0f76c-ls11" as the tag in that screenshot for our image.
The other tag you're confused about is plex's version tag. Our image (different than the official plex image) supports updating to a custom version inside the container by setting an environment variable to a specific plex version. Other options are "latest", "public" and "docker". docker does no updates and you get the version that comes with that image. Public updates to the latest public release on container start and latest updates to the latest beta/plexpass on container start (as long as you're logged in with a plexpass account). Or you can set the version variable to a long plex version and it will update to that. The long plex version for the current stable is "1.15.2.793-782228f99". That's what you would need to enter in as the "VERSION" variable in container settings.
Now as for the other variable for plex downloads, that's the url our image uses to download plex releases. With 1.15, that url changed. So if you enter in a 1.14 version, our image won't be able to download it from the new endpoint, so you need to override that url by passing in the url for old downloads, which is what I listed before. So if you want to update to a 1.15 version, don't specify that url. If you want to update to a 1.14 or earlier, you need to specify that url.
I hope that's clear.
If you simply want to use version 1.15.1.791-8bec0f76c, the easiest way would be to set the "VERSION" variable to "docker" so it doesn't do any updates during container start, and use the image "linuxserver/plex:1.15.1.791-8bec0f76c-ls11" so you'll use the image that comes with that version with no in container updates