The first question is answered in the title you posted.
(1) Flexible Cord. A box shall be supported from a multiconductor cord or cable in an approved manner that protects the
conductors against strain, such as a strain-relief connector
threaded into a box with a hub.
I suppose the second question is in the listing of the cord cap. (receptacle)
I know it is common to see pendants terminated with Russell Stoll or IEC 309 receptacles.
