I think, as in the previous answer, it would be a place where seniors or persons with disabilities reside. Long-term care is, I think, used as a shorthand when we're talking about the bill. However, an institution where seniors reside could be broader, as you've suggested, and my understanding from an operational perspective—again, that's not really my area—is that it is up to the returning officers to contact the local institutions where seniors reside and discuss with them if it would be an appropriate method of polling for them. You are correct in that there is potential for some flexibility in terms of the definition.
I will quickly add that, if only part of the institution has seniors or people with disabilities, it would be the part that was served by the specified polling places.