のうち：between, among

基板面を直交する方向のうち、基板裏側から表側に向かう方向を正方向とし、正方向とは反対方向を負方向とする。

ChatGPT:

How about this translation?

"Among the directions perpendicular to the substrate surface, the direction from the back side to the front side of the substrate is defined as the positive direction, and the direction opposite to the positive direction is defined as the negative direction."

あなた:

Why did you use "among the directions" and not "between the directions" when there should be only two directions that are perpendicular to the substrate surface, i.e., one extending upward and the other downward from the plane of the substrate?

ChatGPT:

Good question. The distinction between "among" and "between" depends on how the directions are conceptually framed.

• "Among" is generally used when referring to three or more things, but it can also be used in a broader sense when discussing a range of possibilities. Since "directions perpendicular to the substrate surface" could be seen as a general category rather than a strictly limited set of two options, "among" might sound more natural in technical contexts.
• "Between" is typically used when referring to a choice or distinction explicitly limited to two alternatives.

Since, in this case, there are only two possible directions (positive and negative along the perpendicular axis), "between the directions perpendicular to the substrate surface" would be a valid and precise choice. If you want to emphasize that the classification is strictly binary, "between" is preferable.