So you can use either is the reason. The internal sensor can only handle a NA build - it's a 1 bar sensor. But you can add a 2.5, 3, 4 or 5 bar sensor on the manifold to handle as much boost as you'd like. The better question is why didn't they install at least a 3 bar map internally in the ECM? If they would have, then the internal sensor could be used for any NA build and any boosted build up to 29 psi.
The sensor you have in the photo is a GM 1 bar MAP 09359409. You can install it in the manifold and plug in the MAP sensor connector and then pick that sensor in the software. Or you can run a vacuum line inside the car and plug it into the port that has the vac line on the ECU. When you do the initial setup with the handheld it's somewhat obvious what's cookin' there. I believe it's also covered in the document that came in the box with the Terminator X IIRC.
Do yourself a favor and don't get hung up on any one thing. Load one of the NA base configs from the SD card that came with the unit that most closely applies to your engine.
The Holley advantage is that the initial setup and startup is the easiest of anything out there. It's a great system that will learn itself in a matter of minutes. The biggest area that you can a crazy issue is with your timing table. Be careful to not make a crazy change that detonates the motor to death. If you need help, then ask - screenshots attached make it easy to see what you're talking about.
Jim
