It's the volume between the lowest part of the overflow pipe and the highest part of the outlet pipe. It can be worked out from the mechanical general arrangement drawing of the tank and details of the connections, neither of which can be seen from here, and double-checked with suitable measuring devices such as a tape measure, dipstick, etc., which need to be sourced close to the tank's location.
__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856