A preset is a slot in a plugin preset list
A preset bank is a collection of presets
I can see no reason why dplug shouldn't be able to maintain backward-compatibility with older versions in the future. However, never say never. This number will be incremented for every backward-incompatible change.
This number will be incremented for every backward-compatible change that is significant enough to bump a version number
Definitions of presets and preset banks.