I’m new at this, so please bear with… I suspect the answer is more about programming style than fundamental characteristics of the Arduino, but I’ll ask anyway.
In a lot of the book examples you use the Arduino output pins as current sinks, rather than sources. For example, when controlling an LED, you wire the board, and set up the logic so that when a pin is LOW an LED will come on, and when that pin is HIGH the LED will be off. I’ve seen it done just the opposite, however, in tutorials from other sources.
Is there a particular reason to use the Arduino output pin as a current sink instead of a current source?