TLS back-ends
When curl is built, it gets told to use a specific TLS library. That TLS library is the engine that provides curl with the powers to speak TLS over the wire. We often refer to them as different "back-ends" as they can be seen as different pluggable pieces into the curl machine. curl can be built to be able to use one or more of these back-ends.
Sometimes features and behaviors differ slightly when curl is built with different TLS back-ends, but the developers work hard on making those differences as small and unnoticeable as possible.
Showing the curl version information with curl --version will always include the TLS library and version in the first line of output.

Multiple TLS back-ends

When curl is built with multiple TLS back-ends, it can be told which one to use each time it is started. It is always built to use a specific one by default unless one is asked for.
If you invoke curl --version for a curl with multiple back-ends it will mention MultiSSL as a feature in the last line. The first line will then include all the supported TLS back-ends with the non-default ones within parentheses.
To set a specific one to get used, set the environment variable CURL_SSL_BACKEND to its name.
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