InstaDMG Variables

Hi All, I'm interesting in finding out more about how InstaDMG uses variables, or more precisely, how scripts that get invoked by InstaDMG use variables. For example, good old $3—from what I have read this is the "current start-up drive" good for using install paths. So my questions are: 1. How is this variable set? I've looked all though the instadmg.bash file and can't seem to find how it is set. 2. If there's $3 then surely there must be $1, $2 and maybe $4, etc. What are these used for if any? 3. Are these variables only used by InstaDMG? I've heard the loginwindow can use $1 and the current user. Excuse the newby question and thanks in advance for any insight.