Volume of the tank used in cubicmetres [m^3] — not in litres, as UDDF uses SI units!
The tank volume can either be given inside <tank>, or inside <tankdata>. If inside <tankdata> a cross-reference is made to <tank> data and if the tank volume is given there, there is no need to indicate the tank volume again inside <tankdata> — indeed, it should not be given again. In case it is, the UDDF file wasn't built thoroughly! Then the last given tank volume (inside <tankdata>) overwrites the "global" value given inside <tank>.
<tankdata>
(see also <tankdata>)
<!-- tank volume 10 Litres -->
<tankvolume>0.01</tankvolume>
<!-- tank volume 12 Litres -->
<tankvolume>0.012</tankvolume>