what does "shell" means?

 From:  Rudy
576.9 In reply to 576.8 
Dear Jesse,
Thank you for adding things that are actually for me learning tools.
Little by little I am getting there.
I am trying to figure out the mill operations as well, if it is worth to get one to start or just send the STL file to a RP company and have my resins done, what about mistakes....I do not want that the company will charge me for a resin that is not correct....all these kind of questions in my head....plus learning to design in MOI (because I really think I love it, more than Rhino....MOI is simpler and very intuitive...I am trying to figure out how it thinks....little by little....

Also because I do not have to forget to earn my daily dough

Thank you,
Rudy
Enjoying your tutorials as well!