In interpreting RD SP2.3 "Identify interface requirements", someone told me that the interfaces can also include man-machine interface, or so-called user interface. But the model says "Interface between functions (or between objects) are identified", at first, I thought that the definition of "interface" in CMMI does not include user interface. But however, in some software development methodologies (e.g. object-oriented), the users are also considered as objects, so, maybe user-interface is also a kind of interface? The same question is also in TS SP2.3 "Design interfaces using criteria", PI SP 2.1 "Review interface descriptions for completeness", and PI SP2.2 "Manage interface."
The interface requirements referred to in Requirements Development (RD) Specific Practice (SP) 2.3, Technical Solution (TS) SP 2.3, and Product Integration (PI) SP2.1 and 2.2 cover ALL interfaces. Keep in mind that you have to do whatever is right for your organization and your projects. If the user interface is important to your project and product, then it should be included in the interfaces covered by the referenced SPs.