You can define your own variables, that will be associated with a specific node. Those variables names can't start with the $ symbol, because by convention it is reserved for default variables. The variables mechanism also supports variables inheritance and overriding in child elements.
To edit your variables, you need to open the Variables tab. On the Variables tab you can see a table with variables, associated with this node. The tab has the following interface:
The user-defined variables editing tab
In variables values you can also reference other variables. Let's rewrite the previous example using variables:
Referencing variables in values