The 'C' include files that define an OFX API are all that are needed by a plug-in or host to implement the API. Most include files define a set of independant 'suites' which are used by a plug-in to communicate with a host application.
There are two include files that are used with nearly every derived API. These are...
ofxCore.h
is used to define the basic communication mechanisms between a host and a plug-in. This includes the way in which a plug-in is defined to a host and how to bootstrap the two way communications. It also has several other basic action and property definitions.
ofxProperty.h
specifies the property suite, which is how a plug-in gets and sets values on various objects in a host application.