The start isolation connection on an ACR is optional, It gets connected to the momentary on position of the starter switch. Its function is to disengage the ACR during starting.

The reason to disengage the ACR during starting is to prevent a drop in voltage that may cause chart-plotters and other electronics to reboot during starting.  The action of starting an engine will always cause the battery voltage to drop.  By disengaging the ACR during starting the drop in battery voltage occurs on the start battery only and the electronics keep working just fine.

Its an optional connection, you can always add it afterwards.  If you don't suffer from that problem then you don't really need to use this feature.

Note that the larger ML-Series ACR's have start isolation connections for two engines, so if either engine is started the ACR disengages.

Start isolation wiring diagram
The diagram above shows the start isolation wiring for one of the ML-Series ACR's, The isolation wire for engine 1 is brown and is connected to the engine start switch.  The isolation wire for engine 2 is green and in this diagram it is not connected.

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