<leafNode name="fwmark">
<properties>
<help>A 32-bit fwmark value set on all outgoing packets</help>
<valueHelp>
<format>number</format>
<description>value which marks the packet for QoS/shaper</description>
</valueHelp>
<constraint>
<validator name="numeric" argument="--range 1-255"/>
</constraint>
</properties>
</leafNode>So the CLI helper tells us it's a 32 bit value, but the range check only allows 8 bits.
iproute2 code revels its a 32bit unsigned integer