The 16.20 release can co-exist with the 15.10.10.00 release and later versions. The active release will be the last release installed. For example, if 15.10.10.00 is installed, then 15.10.10.00 is the active release. Now, if 16.20 is installed after installing 15.10.10.00, then 16.20 will become the active release. The releases can be installed in any order. Installing 16.20 then 15.10.10.00 makes 15.10.10.00 the active release.
There is a script in the bin directory named ttulistproducts_unix.sh that can be used to set the active version if more than one release is installed. This script also indicates the current active version and lists all the products that are 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 will not apply when our software is installed on a shared location. See Third-Party .ENV Files for a description of how to access a particular release.