A sampling technique used when observing lower primates and humans (and probably other stuff, too), when you want to learn about a specific behavior or set of behaviors.
In Sequence Sampling, you focus on a type of behavior, rather than on any one individual or group. Data collection begins when the target behavior (mating, feeding, etc.) starts, and stops when it ends.
Compare this to Scan Sampling, Ad Lib Sampling, and Focal Animal Sampling.