What is "Special endorsement" with respect to Negotiable instruments? What purpose does it serve?

Endorsement of a check  authorizing the payment to an entity other than the entity to which check was drawn.A special endorsement consist of a signature and a statement identifying the entity to which check should be paid.

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An endorsement “in full” or a special endorsement is one where the endorser not only puts his signature on the instrument but also writes the name of a person to whom or to whose order the payment is to be made (also called as special endorsement)

Agree with all answers.

Agreed with the answer Mr. RAMAN NAMPOOTHIRI

Agree with Raman that the special endorsement generally done in favor of third party with additional relevant signatures on the back side of a cheque or a demand draft. The ultimate purpose is to settle the over due amount and a direct credit to the suppliers account rather using middlemen or agents account. 

Agree with Mr. Raman.