RELAY_BEGIN_DIR CELL and the payload

[moris blues]

hi,
i red in a tor-paper:
Clients MUST generate an all-zero payload for RELAY_BEGIN_DIR cells, and servers
MUST ignore the payload..

can someone explain what this means? or does it mean simply that CELL is filled
with zeros.
An attacker could use this not as a conduit for malicious code?

regards
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