IPC (Inter Process Communication) MIDI is a set of two VST plug-ins (a transmitter and a receiver) for sending/receiving MIDI data between different VST hosts - even if they have different architectures (x86 or x64).
Like a MIDI cable between two hosts. Image you have a Cantabile Free 64-bit for your VST's 64-bits and a Cantabile Free 32-bit for your VSTs 32-bits. In a normal situation they can't share MIDI informations.
With this set now you can.
Yry this : in each of your two Cantabile Lite (or SAVIHost, or VSTHost, or whatever you want among the existing hosts, and maybe even DAWs, why not), you place the receiver plug-in in first position (at the top, the very first position), and the transmitter plug-in at the last position (at the bottom, the very last position). Now all the MIDI data that pass in the transmitter at the end of one of your hosts... will be automatically capted by the receiver at the begin of your other host, so that both hosts can talk together.
In this example I placed a transmitter at each end and a receiver at each begin... but nothing forbid you to put them at other positions in the chains of VST"s of each host. And if you don't want a bidirectional communication but only a monodirectional communication you put a transmitter only at the end of one host and a receiver at the begin of the other host.
My example above is an example that, as I wrote, authorize a conversation between 32-bit VST's and 64-bit VST's... a thing that is normally almost impossible except with very rare expensive hosts or with the installation of a bridge (like jbridge within a one host configuration). But you can imagine many other situations (for tests, for backups, for transferts from a studio host to a live host, etc.) allowing such a useful communications between a 32-bit host and another 32-bit host or between a 64-bit host and another 64-bit host... for free.