Octave 3

Sets the octave indicator on the Step Input Keyboard to start at C3. I can’t see any other thing that it affects. I’m not sure if it is simply a guide for the eyes, or if I’m missing the point.

Use step input recording in Logic Pro — Apple Support

Step input allows you to insert MIDI notes when you’re not recording in real time. You can use step input to create note runs that may be too fast for you to play or to replicate sheet music that’s too difficult for you to play.