The Legacy scripts are getting deprecated and will be removed in the first 2022 release, and all installations will swap to the non-legacy version automatically when updated.
Legacy Non minified & Legacy Minified - Same as above, but using the legacy jQuery versions of the scripts
Legacy Non minified scoped & Legacy Minified Scoped - More robust versions with a scoped jQuery script
Which script source should you use?
Usually the best is used the Minified version, from both the legacy and non-legacy options. Even if you are using a cache plugin with asset minification feature, using the minified version should cause no issues - as further minification is not applied in most cases.
If you are experiencing issues with a cache/minify plugin, try the Non-minified method instead.
What there are issues, and only the Legacy versions work for me?
You should definitely report that to us via our support. The legacy scripts are going to be removed, so please make sure to report any issues to us, so we can patch them as soon as possible.
This option is only available when a non-legacy source is selected.
Classic - The plugin core scripts are merged into a single file (+ a prerequesite file). This file contains all of the core functions, but most likely it contains code you may not need. This option is the most robust choice, will most likely work in all cases.
Optimized - The plugin will enqueue more script files, however only the ones definitely required. It can save a lot of page loading time.
Optimized asynchronous - Only a single script file is enqueued, then this file will request the rest of the required script files in the background. It should be the fastest.
Which one should you choose?
Choose Optimized, when you are using a cache/minify plugin, CDN or any other cache related solution. The cache plugijn will take care of the rest.
Choose Optimized asynchronous, when you have a minimalistic setup, not really using many other plugins/scripts, and you don't have an effective minify/cache solution.