A plugin needs to define it's parameters during a describe action. It does this with the OfxParameterSuiteV1::paramDefine function, which returns a handle to a parameter description. Parameters cannot currently be defined outside of the plugins describe actions.

Parameters are uniquely labelled within a plugin with an ASCII null terminated C-string. This name is not necassarily meant to be end-user readable, various properties are provided to set the user visible labels on the param.

All parameters hold properties, though the exact set of properties on a parameter is dependant on the type of the parameter.

A parameter's handle comes in two slightly different flavours. The handle returned inside a plugin's describe action is not an actual instance of a parameter, it is there for the purpose description only. You can only set properties on that handle (eg: label, min/max value, default ...), you cannot get values from it or set values in it. The parameters defined in the describe action will common to all instances of a plugin.

The handle returned by OfxParameterSuiteV1::paramGetHandle outside of a describe action will be a working instance of a parameter, you can still set (some) properties of the parameter, and all the get/set value functions are now useable.