A Series of signals issued by the Admiral in charge to his subordinates outlining his intent and how he will conduct his operations.
The OPGEN (OPerations GENeral) Signal is the overarching signal, setting the scene and covering most topics in broad brush strokes.
OPTASKs (OPerational TASKings) provide much more detail in each warfare sphere, and provide the specifics every Ship will need to be able to take their place safely and efficiently in a Task Group. They can be drafted by the Admiral in charge (or his Staff!), but are generally delegated to the Officers responsible for a particular warfare sphere: the Senior Destroyer will typically be the Anti Air Warfare Commander (AAWC) and will draft the OPTASK AAW; the Senior Frigate will be the Anti Submarine Warfare Commander (ASWC) and probably the Anti Surface Warfare Commander (ASuWC), and hence responsible for the OPTASKs ASW and ASUW respectively.
This signals are pre-formatted to a NATO Standard, allowing their use across the Alliance, and by other Allies (RAN, RNZN etc) who utilise our signal formats.
As we have moved on, OPTASKs can be found on subjects as disparate as meteorology and information management!