CLSNA' LamontCentral Forum

a great web site that you can get great advise and find many vendors is called

Xmarksthescot.com

To answer your question yes it is the number of yards of tartan. the difference is in the pleats, 5 yd has less pleats and they are not as deep. I have one of each and think at first glance nobody can tell. The 8 yard has more swish but I doubt anybody is checking me out for that  LOL

That web site is where I got my first questions answered and found both vendors for my kilts.

When they refer to double width, as in 4 yards double width, it means they can make either 2 four yard casual kilts, or 1 eight yard kilt. They split the fabric and sew it together to get a eight yarder. The more material, the more and deeper pleats which gives the kilt a nicer swing and more formal look. The downside to 8 yarders is they weigh more and tend to cause you to heat up quicker ( which isn't that bad if you live in a cold climate ). 8 yarders also stand down better on windy days.
Casuals ( 4-5 yarders ) are better for warmer climates,less formal affairs, or daily wear.
Each has its ups and downs, so it all comes back to your needs. I hope this helps.