Without seeing the CDF it's hard to know precisely how it's been done in the particular (unnamed) PDK that you're using, but my guess is that it's being done by a callback. The callback is not on the destination parameter (ad, as, pd, ps etc), but on the source parameters (w, l etc). Did you check there?
Even if you see the callback, you may not be able to see the source of the callback because it may be either encrypted or in a binary context file.
In reply to Andrew Beckett:
In reply to shushanik:
Or ask the provider of the PDK for an explanation of the relationship...