Expression for business key in a CallActivity: why # and not $?

[aravindhrs]

@fml2, you can refer the below post for the differences between usage of # vs $ in expressions.

'$' vs '#' usage in expressions

The symbols were confusing whether to use ‘$’ or ‘#’ in expressions. Lets say i have a process variable “assignee”, in bpmn file should i have to mention ${assignee} or #{assignee}. In forums, i saw many of them posted using ‘#’ in expressions. Which one is valid and when to prefer one against another?