New in version 3.30.

This is a CMake Environment Variable. Its initial value is taken from the calling process environment.

Specify the default value for the file(DOWNLOAD) and file(UPLOAD) commands' TLS_VERSION option. This environment variable is used if the option is not given and the CMAKE_TLS_VERSION cmake variable is not set. See that variable for allowed values.

This variable is also used by the ExternalProject and FetchContent modules for internal calls to file(DOWNLOAD) and git clone.