I don't recall who said it but :

"Friends are the family we choose for ourselves."

Traditional clans were related families. You were born to the clan by blood and had no choice in the matter. As far as XMTS, you CAN'T be born into or have blood-ties so you have to CHOOSE to be a member. We are a "family" (and therefore clan) made up of people who choose to be in this family. That's how I read the motto and feel it is very fitting considereng how one becomes a member of this "clan".