Symbols and collections support metadata, a map of data about the symbol or collection. The metadata system allows for arbitrary annotation of data. It is used to convey information to the compiler about types, but can also be used by application developers for many purposes, annotating data sources, policy etc.
An important thing to understand about metadata is that it is not considered to be part of the value of an object. As such, metadata does not impact equality (or hash codes). Two objects that differ only in metadata are equal.
That said, metadata and its relationship to an object is immutable - an object with different metadata is a different object. One consequence of this is that applying metadata to a lazy sequence will realize the head of the sequence so that both objects can share the same sequence.