Forerunner symbols

In the Halo Universe, there are many Forerunner Symbols, pictograms made by the Forerunners. These symbols can be found above doors, on Elite armor, virtually anywhere on the Installations, and in the Covenant.

According to Bungie, namely Joseph Staten, the blue and red signs mean a 'safe zone' as in whether or not the Flood are there.

Use
The exact nature of Forerunner writing remains a mystery. While symbols obviously have meaning attached, the exact sentence structure, if any, remains unknown. It is more likely that these glyphs are a system of logograms, rather than a alphabet or syllabary. There also seems to be more dimensions to Forerunner writing than traditional methods, with the individual glyphs holding multiple meanings, likely making them entirely contextual - when combined with different glyphs, a symbol might have radically different meanings, making individual glyphs pictograms or ideograms.

Other symbols found on Forerunner relics and structures are even less obvious in their meanings. They are obviously decorative in nature, but they are often larger and more elaborate in nature than their glyphs, and may depict specific shapes and forms, and may have more in common with petroglyphs than logograms.

Symbol Analysis
Taking a closer look at the symbols and where they were found and what has been said by Bungie we can deduce the following:

All the others are found in various Forerunner structures, mostly on the level, 343 Guilty Spark, Silent Cartographer, and The Library.

Cropped Symbols Analysis
Several of the symbols are variations of each other. There are a total of 16 unique symbols; the other 8 are rotations or croppings of the 16 main symbols. Of note is the fact that the cropped versions may not have been intended to be cropped. They appear on a wall or something, similar to the non-cropped version, but they are placed (as decals) in such a way that part of their texture extends past the edge of the poly that they are on, thus clipping it off. But, in sake of being thorough, they have been included as separate symbols.

One could speculate that the orientation of the symbols changes their meaning in some way, like verbs in English (run - running - ran - runner - etc.).

Numbers
Only the Forerunner symbols for numbers one through five are known. It is probable that they also have a symbol for 0, but they may use a base 6 system and not have any more numbers.

Installation Symbol
In Halo 3, a number of details about the seven installations arise. In the Citadel room, where Truth is preparing to fire the Installations, each Installation is shown in detail: its status, structure and a symbol to recognize each Installation. Some notes say that there are two versions of each symbol, one where you enter the ring room and the view from the pedestal.

Shield World?
Several symbols have appeared throughout Halo 3 and on the cover of Ghosts of Onyx. In the book it was described as the symbol of Shield World but the symbol is seen ontop of 343 Guilty Spark and behind his "eye." It's also seen in the terminals.

Trivia

 * It is unknown if the Mark of Shame is a Forerunner Symbol.
 * The symbol for 0 is the same as the avatar for AdjutantReflex.
 * There is at least one Forerunner Symbol that match the Marathon Symbol.