CURLS (with S) which uses our statically linked CURL library. And it includes SSH and SSL libraries, so it has no dependencies. You can use the plugin and no other DLLs are required.
CURL (without S) does the same, except that it comes without a built in library. You can load whatever CURL library you want. For example you can use the one which comes with macOS or Linux. Or you provide your own dll/dylib/so files. Those versions may be with or without SSH or other features.
For 7.1 plugins we made sure both sets of classes have the same features and match in classes. So we got FileInfo class for CURL part, too.
If you are interested to use native SSL libraries on Mac or Windows and ditch OpenSSL, we have later special CURL Libraries for you. They work well with CURL plugin, but you have to include them with your application and load them at runtime.