Which measurement involves assessing the lateral width of the underarm seam?

The measurement that involves assessing the lateral width of the underarm seam is indeed chest width. This measurement is taken from one side of the body to the other at the underarm area, providing an accurate indication of the garment's fit across the chest. It is a crucial dimension for ensuring that the clothing fits comfortably without being too tight or too loose in this area, which is particularly important for movement and overall comfort.

In contrast, sleeve length measures how long the sleeve is from the shoulder seam to the end of the cuff, sleeve width gauges how wide the sleeve is around the bicep area, and shoulder width reflects the distance from one shoulder seam to the other across the back. These measurements serve different purposes in tailoring and clothing design, but chest width specifically focuses on the horizontal dimension at the underarm seam.