presentation of information about the affiliation of the object. If colour is not used, the fill is transparent. A very few icons have fills of their own, which are not affected by affiliation. The icons themselves, finally, can be understood as combinations of elementary glyphs that use simple composition rules, in a manner reminiscent of some ideographic writing systems such as Chinese. The standard, however, still attempts to provide an "exhaustive" listing of possible icons instead of laying out a dictionary of component glyphs. This causes operational problems when the need for an unforeseen symbol arises (particularly in MOOTW), a problem exacerbated by the administratively centralised maintenance of the symbology sets. When rendering symbols with the fill on, APP-6A calls for the frame and icon to be black or white (as appropriate for the display). When rendering symbols with the fill off, APP-6A calls for a monochrome frame and icon (usually black or in accordance with the affiliation colour). NATO symbols can also be rendered with fill off using a frame coloured according to affiliation and a black icon, though this is not defined in any APP-6 standard. Allegiance and affiliation APP-6 colour representation The concept of affiliation does not appear in the original APP-6 as these were not introduced until APP-6A. Instead, the original APP-6 described a series of "colour representations" with the purpose of distinguishing friendly and enemy elements. Multi-colour representation: Blue or black for friendly icons Red for enemy icons Green for man-made obstacles (friendly or enemy) Yellow for chemical, biological, radiological or nuclear events Other colours to be established in a map legend tri-colour representation: Blue or black for friendly icons Red for enemy icons Green or yellow for man-made obstacles (friendly or enemy) and for chemical, biological, radiological or nuclear events two colour representation: Blue, Green, or black for friendly icons Red for enemy icons One colour representation: Single line border for friendly icons double line border for enemy icons, and unbordered icons were labeled with "EN" to the lower right corner APP-6A affiliation Affiliation refers to the relationship of the tracker to the operational object being represented. The basic affiliation categories are unknown, friend, neutral, and hostile. In the ground unit domain, a yellow quatrefoil frame is used to denote unknown affiliation, a blue rectangle frame to denote friendly affiliation, a green square frame to denote neutral affiliation, and a red diamond frame to