Filter jargon easily can lead to gross misapplication and misinterpretation of the 'data'.

The MAX. Flow is what such a filter assembly with max. µM cartridge will pass at MAXIMUM Housing pressure rating .... i.e..: how much this housing assembly will deliver wursthen pressurized to the published 30 PSI maximum (burst) pressure .... the pressure at which the housing will leak / rupture / or the filter cartridge will 'collapse' .... such cartridges can 'easily' collapse at high vacuum about -5-10 psid.

Go to page B34 of http://www.parker.com/literature/Racor/7480H_Catalog_Marine_Fuel_Filtration_April_2010.pdf for the nominal operational flow data.
The very extreme max that a simple single stage marine diesel engine lift pump will develop is about 2" Hg. vacuum or about -1.0 psi.
Go to the flow rate chart (differential pressure vs. gallons per HOUR), read/extrapolate where 1.0 psi differential pressure 'crosses' the GPH axis .... about 60 gallons per HOUR at 1.0 psi for MAXIMUM FLOW RATE.
Now go back to your 'historical' operating fuel system gage pressure of normal 'vacuum' and go back to the chart and read ~5 gph .... which will be for the filter assemblies 'maximum' for a NEWLY installed and ***CLEAN*** filter cartridge... for YOUR maximum operational limit.

Parker/Racor 'used to' publish much better flow data (∆P vs. GPH) so dont take what you see on Page B34 as 'gospel' as its seemingly only a rough 'approximation'.