Each version of the Ion specification defines the corresponding system symbol table version. Ion 1.0 uses the "$ion" symbol table, version 1, and future versions of Ion will use larger versions of the "$ion" symbol table. $ion_1_1 will probably use version 2, while $ion_2_0 might use version 5.

Applications and users should never have to care about these symbol table versions, since they are never explicit in user data: this specification disallows (by ignoring) imports named "$ion".

Here are the system symbols for Ion 1.0.