String expansion is a feature that allows strings to dynamically define their value at run time. This expansion is done via the following syntax:
This feature is used to create policies that refer to concepts rather than to specific values. For example, a policy can be created that refers to the User-Name in a request, via:
This string expansion is done only for double-quoted strings and for the back-tick operator, as described above. Unlike other languages, there is no way to define new variables. All of the string expansions must refer to attributes that already exist or to modules that will return a string value.