Output Multi Part - how does it work?

There are a number of inputs on Output Multi Part - but how are they assigned and labeled in an EXR? I need to add several mask channels to an EXR, but it is unclear how these are organized in the EXR.

Silhouette does not support multi-channel EXR files, but does support multi-part files. Multi-part image files can contain a number of separate, but related, images in one file. Access to any part is independent of the others; in particular, no access of data need take place for unrequested parts.