HRC relates to night-time needs (in addition to day-time needs), so to award that, they need significant care throughout both the day and night. MRC is care throughout the day, with little needed at night. (I think HRC is needing care most nights, so eg 5 out of 7).
My DC was awarded MRC as although her day time needs are fairly significant, she probably only needs care 2 out of 7 nights (apart from the last week 🤔 Let's hope it's not the start of a new pattern)
Does that follow for you? Does your child who received HRC need more care at night than the other?