Nearly all plug-ins have some sort of parameters that control their behaviour, the radius of a circle drawer, the frequencies to filter out of an audio signal, the colour of a lens flare and so on.

Seeing as hosts already provide for the general management of their own native parameters (eg: persistance, interface, animation etc...), it would make no sense to force plug-ins to do this all themselves.

The OFX Parameters Suite is the means by which parameters are defined and used by the plug-in but maintained the host. It is defined in the ofxParam.h header file.