Total Pressure Drop - The calculated pressure drop through the exchanger based on what conditions the exchanger is designed for (i.e. specified flows, densities, viscosities, etc.)
Allowable pressure drop - A specification provided by a process design engineer that is the maximum allowable design pressure drop (or total pressure drop) for that exchanger.
For example, I have a process where I have a pump that produces 50 psig head, going through an cooler to a tank at 10 psig. A control valve is designed for 15 psig pressure drop, and piping to the tank is expected to have 15 psig pressure drop. Thus there are 10 psig available for the cooler. The process design engineer could specify an allowable pressure drop of 10 psig in this case (though you would probably want to specify a little bit less).