$ID
References:
Any identifier (starts with a letter and can follow with letters or numbers will be initialized as an $ID. And an $ID can be used for many things, including associating a value. Optionally, the '@' symbol is used to de-reference an $ID to retrieve the data stored - which is useful when being used in a location that requires and $ID (such as a label). When used in a location that requires a value, an $ID will be automatically dereferenced.
Example of using an $ID as a variable:
> x = "hi"
> x
hi
> x + " test"
hi test
> {x:"test"}
{"x":"test"}
> {@x:"test"}
{"hi":"test"}