GammaLib

Here some information from the FSSC web pages:

Internally the IRFs for each event class contain a set of IRF parameterizations for individual event types. The event types are organized in partitions that split the events within a class according to different criteria. Whereas previous data releases provided only a single partition based on the conversion plane of the reconstructed track (FRONT versus BACK), the Pass 8 data release defines three event type partitions: FRONT/BACK (two types), PSF (four types), and EDISP (four types).

The Science Tools support analysis with both individual event types as well as any superset of event types within a partition (e.g. PSF2+PSF3 or FRONT+BACK). In the case that the data selection is a superset of event types within a partition, the Science Tools will apply the response function appropriate for the sum of those event types. In the typical usage scenario in which all events within a class are selected (equivalent to evtype=3) the Science Tools will apply the response function for the sum of FRONT and BACK events.

Event types within a partition are mutually exclusive and therefore an event can only belong to one of the types in each partition.

Looking a bit in the actual code, it seems already pretty well prepared for general event type handling. Each response component has a size() method and the different event types can be accessed via an index, so the only thing needed may be related to loading the right number of response functions.