I understand that the system of combination of high-cost medical care and long-term care applies to those in the same household. But what does same household mean in such cases?

For the purposes of this system, a household refers to a unit of members of the same health insurance. For this reason, those who live in the same household but are members of another health insurance, such as National Health Insurance or the Medical Care System for the Advanced Elderly, are treated as members of separate households. If a person was a member of the same household for part of the calculation period because of his or her withdrawal, his or her copayment for that period will be included in the combination.