Those carried by legal or other documents? (6)

I believe the answer is:

riders

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'documents?' is the definition.
The definition and answer can be both related to communication as well as being plural nouns.
Maybe they are linked in a way I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for riders that I've seen before include "Supplementary clauses" , "As additions to bill" , "... To The Sea (J.M. Synge)" , "they mount up" , "qualifications" .)

I've seen this clue in the Evening Standard.

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