Any linguistic annotation is annotation of some data.
We call signal such a source of linguistic information. Such a document can be a text file, an audio recording, a scanned picture or a video. Not everything is equally meaningful in any given signal , some parts being more interesting than others. Interesting regions of signals have to be delimited in a way that allows to unambiguously determine the boundaries of such regions. Anchors can be considered as coordinates in the signal plane or as events in the signal data.