The download must be installed somewhere and installation begins by extracting the downloaded archive. The location of the extraction is the installation base directory. In the online documentation, this directory is referred to by the symbol [INSTALL_BASEDIR].
The typical answer, at the moment, is the user home directory.
In Linux, this is best typed
although in most cases, the shorter
will work.
In Windows use
The subdirectory principia of [INSTALL_BASEDIR] is the [PRINCIPIA_HOME].
Wiki: INSTALL_BASEDIR
Wiki: Installing Principia
Wiki: PRINCIPIA_HOME