Direct quotation: 5.4.3 p.115
"He must also have a shield, suitable terrain, or a Melee weapon.
Some weapons may only be used to parry with a certain
percentage of the wielder’s OB."
This is the 5500 RMSS Main Rule Book passage.
• He may also have a shield, suitable terrain, or a Melee
weapon. Some weapons may only be used to parry with
a certain percentage of the wielder’s OB (see the Defensive
Capabilities Table T-3.6). Note that he may also
parry attacks if making a martial arts-type attack (though
he has no weapon except his body).
I always just have to shake my head when more recent versions of a game get
less clear.