Sub-sessions are one of the most powerful new features in Cantabile and provide a way to save named states within a session file.
For example, suppose you have a performance that has several instruments that you need to quickly switch between. Often loading these plugins at the time they're required can be time consuming. With sub-sessions, you can load all three instruments into one session and save three sub-sessions within that session - with one instrument enabled in each.
In the following screen shots, the image on the left has the Ivory sub-session selected while the image on the right has the mda ePiano sub-session selected. Note the difference in bypass states on each plugin.

Sub-sessions can be used to optionally control many aspects of a session, including:
Also note that sub-sessions are integrated with the session list.
Sub-sessions are available Cantabile Performer edition only.