The way this works as far as VST is concerned: a plugin has a set of programs (the number of programs is determined by the plugin) and all these programs together constitute a bank.
The host (Cantabile) can ask the plugin for a copy of either the bank, or the current program which gets returned to the host as a chunk of memory that the host can later pass back to the plugin to reload it.
The host has no way to interpret this chunk of memory - the plugin is the only thing that understands what’s in that chunk. Typically these chunks get saved to disk as part of the song and passed back to the plugin when it’s reloaded.
What exactly the plugin returns in these chunks of memory is completely up to the plugin, but when saving the bank it’s supposed to represent the entire state of the plugin.
When a plugin indicates that it only supports 1 program, Cantabile simulates a bank of programs by saving 128 different bank chunks - ie: switching presets actually causes Cantabile to reload a previously saved chunk; as opposed to simply telling the plugin to switch program. This is called “Pseudo Presets”
To complicate things a little more, some plugins don’t support chunks at all. For these plugins all the plugin’s exported parameters are saved and restored by the host. Not many plugins use this approach.
All Cantabile would have done is told the plugin to “load this chunk” where the chunk means whatever the plugin wants it to mean. The plugin might actually save the name of an external file to reload, or it might store a copy of all the parameters, or it might store an index into a hard coded bank. The host has no idea.
Typically though, it will be a copy of all parameters for all programs.