+A21 often signifies a specific designation within a broader framework. It can denote a [Project Code](/wiki/Project_Code) or a particular [Component](/wiki/Component) in an intricate structure. Its meaning typically derives from the context in which it is used, serving as a unique identifier.
+## See also
+- [Identifier](/wiki/Identifier)
+- [Classification](/wiki/Classification)
+- [System](/wiki/System)
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