in my 8 years of kilt wearing the main things for modern I find to be are when its more ..... dressed down. t-shirt untucked hose slid down to the boot top. soo i guess modern to me would be casual vs formal and semi formal. Matirial of kilt ..... depending on yardage may also factor into it. Ultimatly as a wise auld coot once told me. It's up to you to figure your oun differance between the two. Trust yourself.