I think you've got it right, Alex, but this calls for an explanation of the phrase numeric scalar. The word numeric just means a number, and not a binary or a string (letters). The word scalar means just a single value, not a vector or a matrix.
The number 10 is numeric, and it's a scalar. The fact that it has units associated with it is okay, because the coordinates of P also have units associated with them. UTM is in meters, and your buffer length is also meters. As long as the units of your buffer are the same as the units of the P coordinates, you're alright.
To check, I would plot P and your buffered version of P on the same plot like this:
P_b =polybuffer(P,'lines', 10);
Then zoom in and visually confirm that the two polygons are separated by 10 m.