Septs and Associated Families

[Bryan]

I didn't know where to stick this, so I thought the Heraldry Forum would work. My question is, what is the difference between a sept name and a "name associated with" a particular clan? Are all surnames listed under a certain clan entitled to wear that clan's tartan?

The reason I ask is maternally I am Scottish through the surname Butters. Butters is a "name associated" with Clan Murray. I asked a very dear friend of mine who is from Scotland and is coincidentally a Murray, if I could wear the tartan of Murray of Atholl as a sign of my maternal heritage. He said, of course, that I could since my family was associated with his particular clan. I'm still confused though. Why distinguish between associated families and septs? I assume that because a family is ASSOCIATED with that clan in some way, they can wear that clans tartan. Right? Help.