What is this workflow expression syntax?

[Guido1982]

I'm researching the possibilities of automated price modification and was reading this blog. In there I came across this workflow syntax

Code:

<map>
<expression>if '$[bill_country]' == 'Spain' then 10 else 20 end</expression>
</map>

I was wondering about the '$[bill_country]' syntax. I've never seen that before. Where can we use that (and where can't we?)