efficient transmission of symbology information through the use of a standard methodology for symbol hierarchy, information taxonomy, and symbol identifiers. APP-6A recognises five broad sets of symbols, each set using its own SIDC (Symbol identification coding) scheme: Units, equipment, and installations Military operations (tactical graphics) METOC (meteorological and oceanographic) Signals intelligence MOOTW (military operations other than war) Units, equipment, and installations consist of icons, generally framed, associated with a single point on the map. All sorts of graphical and textual modifiers may surround them, specifying categories, quantities, dates, direction of movement, etc. Tactical graphics represent operational information that cannot be presented via icon-based symbols alone: unit boundaries, special area designations, and other unique markings related to battlespace geometry and necessary for battlefield planning and management. There are point, line and area symbols in this category. Meteorological and oceanographic symbology is the only set not under the standard's control: rather, they are imported from the symbology established by the World Meteorological Organization. The signals intelligence and military operations other than war symbology sets stand apart from Units, Equipment, and Installations although they obey the same conventions (i.e., they consist of framed symbols associated to points on the map). They do not appear in APP-6A proper, having been introduced by MIL-STD-2525B. Symbol composition Most of the symbols designate specific points, and consist of a frame (a geometric border), a fill, a constituent icon, and optional symbol modifiers. The latter are optional text fields or graphic indicators that provide additional information. The frame provides a visual indication of the affiliation, battle dimension, and status of an operational object. The use of shape and colour is redundant, allowing the symbology to be used under less-than-ideal conditions such as a monochrome red display to preserve the operator's night vision. Nearly all symbols are highly stylised and can be drawn by persons almost entirely lacking in artistic skill; this allows one to draw a symbolic representation (a GRAPHREP, Graphical report) using tools as rudimentary as plain paper and pencil. The frame serves as the base to which other symbol components and modifiers are added. In most cases a frame surrounds an icon. One major exception is equipment, which may be represented by icons alone (in which case the icons are coloured as the frame would be). The fill is the area within a symbol. If the fill is assigned a colour, it provides an enhanced (redundant)