The EU6500is will provide up to 45.8 amps at 120 volts, but it's spread out among the various receptacles. You can't get a full 45.8 amps out of a single receptacle.
First, assume you only need to power 120 volt devices. Keep the Voltage Selector Switch in the 120V ONLY position. This routes the maximum amount of current to three receptacles (1,2,3). #4 is usually only used when you need to make 240 volts.
Next, determine the power required for each device. Let's say you have an air conditioner that requires 25 amps to start; you'd want to plug it into receptacle #1, since it can deliver up to 30 amps. You would NOT want to plug it into receptacle #3, since it only makes 20 amps.
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Caveat: I work for Honda, but the preceding was my opinion alone.