The rate interval is the period of time over which the age label remains the same.
"Curtate duration at death" is the duration the policy is in force until the life dies, rounded down to the nearest integer. This implies a policy year rate interval - starting on the policy anniversary where lives attain a certain age label. We classify them as age x at the start of the policy anniversary.
"Age at the birthday in the calendar year of entry" into the policy - implies that in the calendar year of the first policy anniversary (i.e. from 1st January to 31st December) - policyholders were aged [x-1, x] i.e. "x next birthday", if we classified them as age x at the start of the policy anniversary.
Thus the rate interval implied here is:
"Policy year rate interval, starting for lives classified age x, on the policy anniversary where they were aged x next birthday."
You can also determine this diagrammatically which would help in the exam :)