I think you have it wrong, check again. It may say the maximum loop resistance is 600 Ohms @ 24 VDC, this is because at 20 mA your transmitter needs at least 12 V to operate.
Assuming your PLC or DCS is 250 Ohms (typical), you would be able to add 1 more 250 Ohm load, e.g. a chart recorder, and still have 100 Ohms available for cable resistance.
That's the advantage of a 4-20mA signal, for a given pressure input the signal stays the same from 0 to 600 Ohms, the transmitter just drops less voltage to compensate.
BTW if you need to add more resistance it's usually possible to boost the supply voltage to 48 or higher, allowing you to send a signal for miles.
The manufacturer may have assumed that you have a 24V dc supply in your loop and is trying to say that once you have allowed for the voltage required by the transmitter, you have enough voltage left to drive an external loop resistance of 600 ohm (load + cable etc. It is just another example of an ambiguously written datasheet
