Picking up where we had left off at Vladivostok, we discussed verification of underground nuclear tests that each side could conduct, and also the rules for counting MIRVs — how to tell whether a missile had a single or a multiple warhead. There the Soviets granted a concession. If they placed a MIRV in a certain location, then we had the right to assume that every missile in that field had a MIRV capability.
Source: wiktionary