The 17.10 release can co-exist with the 15.10.10.00 release and later versions. The active release is the most recent release installed. For example, if 15.10.10.00 is installed, then 15.10.10.00 is the active release. If 17.10 is installed after installing 15.10.10.00, 17.10 becomes the active release. The releases can be installed in any order. Installing 17.10 then 15.10.10.00 makes 15.10.10.00 the active release.
A script in the bin directory named ttulistproducts_unix.sh can be used to set the active version if more than one release is installed. This script identifies the current active release and lists all the products installed on the system. The difference between the active release and the non-active release is that the bin directory will be on PATH, and symbolic links created into /usr/lib and /usr/bin by default. The Install Configuration File section indicates how to change the default behavior.
The notion of an active release does not apply when TTU software is installed on a shared location.