In the specification, section 8.2.1 "Enclosing Elements", it says:

  Evaluation of a WS-BPEL expression or query will yield one of the following  ...:
  - a single XML infoset item ...
  - a collection of XML infoset items ...
  - a sequence of CIIs for simple type data ...
  - a variable reference: e.g. <from>$myFooVar</from>

English is not my mother tongue, but here I read "yied" as "evaluate to". In this understanding, I do not understand the variable reference: Why would the expression "$myFooVar" not result in either single XML infoset item or sequence of CIIs?

What exactly is meant by "variable reference"? I tried to categorize the types of queries and expressions, but I cannot see which kind of expressions that should yield a variable reference.

The example <from>$myFooVar</from> must be a General Expression as listed in section 8.3 "Expressions", because of the from-tags.

In section 8.3.5 "General Expressions", is says that general expressions "results in any XPath value type (string, number, or Boolean)." (I guess node-set should be listed here too, but that is another issue)

However, in section 8.4 "Assignment" under "Expression variant" is says:

  This computed value of the expression MUST be one of the followings:
  - a single XML information item other than a CII ...
  - a sequence of zero or more CIIs ...

So I cannot figure out how the general expression "<from>$myFooVar</from>" should evaluate to a variable reference or what a "variable reference" in this context is defined as.

I am looking forward to hearing your opinion. Thanks.