Meaning of "which is expressed to be so payable"

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A promissory note, bill of exchange or cheque is payable to order, which is expressed to be so payable, or which is expressed to be payable to a particular person, and does not contain words prohibiting transfer of indicating an intention that it shall not be transferable.

Here, what is the meaning of `which is expressed to be so payable'

Could anyone explain it?
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[Stew.t.]

Hi

The phrase means to "be paid by transfer (exchange) or cheque".

The source text here is interesting, where is it from?. It has a density that is so typical of legal or financial English. So thick your brain turns to mush thinking about it.
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