In CallbagKit a sinker subscribes to a source. Then that sinker reacts to whatever item or sequence of items the sources emits.
There are three types of sources in general.
Listenable source which will emit items whenever there is an item available to be emitted.
Pullable source emit items only upon request from sinker, an item per request.
Single source is actually a Listenable source, but will only emit one item whenever it is available to be emitted.