Kaylani used fabric measuring 5 yards in length to make each suit for a men's choir.

Choice D is correct. It’s given that the equation $y-5x=6$ represents the relationship between the number of suits that Kaylani made, $x$, and the total length of fabric she purchased, $y$, in yards. Adding $5x$ to both sides of the given equation yields $y=5x+6$. Since Kaylani made $x$ suits and used $5$ yards of fabric to make each suit, the expression $5x$ represents the total amount of fabric she used to make the suits. Since $y$ represents the total length of fabric Kaylani purchased, in yards, it follows from the equation $y=5x+6$ that Kaylani purchased $5x$ yards of fabric to make the suits, plus an additional $6$ yards of fabric. Therefore, the best interpretation of $6$ in this context is that Kaylani purchased $6$ yards more fabric than she used to make the suits.

Choice A is incorrect. Kaylani made a total of $x$ suits, not $6$ suits.

Choice B is incorrect. Kaylani purchased a total of $y$ yards of fabric, not a total of $6$ yards of fabric.

Choice C is incorrect. Kaylani used a total of $5x$ yards of fabric to make the suits, not a total of $6$ yards of fabric.