I/DD stands for intellectual and developmental disabilities. The slash is the point of the phrase. It names two overlapping service categories together, so a program is not read as covering only people with an intellectual disability. Agencies, waiver names, and federal offices use the pair when they mean the population this whole service system is built around.
An intellectual disability is one kind of developmental disability. A developmental disability, in program language, is the broader category. Putting both words in the name is how a state says its waiver, its agency, or its report includes that broader group. It is not a second diagnosis stacked on a first one, and it is not a clinical score. Medical sources, including AAIDD, publish clinical definitions. This entry does not restate them.
You will see the pair written several ways: I/DD, ID/DD, and intellectual/developmental disabilities. Style differs by agency. The population they are pointing at is the same one, which is why the no-slash acronym IDD can refer to the same offices and the same waivers. Punctuation is not a different eligibility group.
What the letters do not do is enroll anyone. A state's level-of-care screen, written into the waiver, decides who can receive services. Two people can both be described with this phrase in ordinary speech and only one of them can be on a given waiver, because the waiver has slots, ages, and functional criteria of its own.
News on this desk uses I/DD the way budget documents do: as the name of the service population. When a story is about a specific waiver, it should use that waiver's name. When a story is about the field, this is the paired term, and the definitions of intellectual disability and developmental disability are the two halves.