It's not very clear I have to say so here's my take on it.
The connection called "system load" is an unregulated output that I believe is intended to feed some other supervisory chips that might control the MAX1873 in other ways such as disabling the chip (see ICHG/EN pin) or monitoring the current fed into the battery. I say this because the input voltage for the charger circuit is 9V to 28V and it does not make sense that this is used for your load.
It makes sense that your load is connected to the battery as in pretty much any other battery powered application. I'm not 100% but that's my take on it after a few minutes trying to make sense of the topology.
I will add this too. The connection called "system load" will also receive battery voltage minus the volt drops of the external P channel FET and the current limit resistor so it looks like this connection will always receive power. In fact the FET volt-drop could be quite small because I see no reason why the FET should not remain active when there is no external input supply.
This answer is speculation in the main part because the data sheet doesn't seem to really say what is going on.