When drawing a 10 mm single-stroke capital letter in vertical style, what does the distance of 10 mm between the horizontal parallel guidelines represent?

According to the lettering and numbering guidelines in BIS SP: 46-2003, match the width (W) of uppercase letters to the correct group of capital letters (where 'h' is the height of the letter): Width (W) Group: (A) 5/14 h (B) 6/14 h (C) 7/14 h (D) 9/14 h Letter Group: (I) W (II) A, M, Q, V, X, Y (III) B, D, G, H, K, N, O, P, R, S, T, U, Z (IV) C, E, F, L

When creating 10 mm single-stroke capital letters in vertical style, what does the 10 mm distance between horizontal parallel lines represent?