Telemetry intelligence (TELINT) is a subdiscipline of FISINT which is concerned with missiles and other remotely monitored devices sending back continuous streams of data about their location, speed, engine status and other metrics. This data can provide information on the performance of the missile and especially its throw-weight, i.e. the potential size of its warhead\s.
Strategic significance of TELINT
TELINT is one of the "national means of technical verification" mentioned, but not detailed, in the Strategic Arms Limitation Treaty (SALT) between the US and USSR. The (SALT I) treaty language "the agreements include provisions that are important steps to strengthen assurance against violations: both sides undertake not to interfere with national technical means of verification. In addition, both countries agree not to use deliberate concealment measures to impede verification." refers to, in part, a technical agreement not to encrypt strategic test telemetry and thus impede verification by TELINT.