actually the mk VI is used in halo 2 and 3. the variants are strictly from the novels and possibly will be featured in the upcoming Halo Wars game. ODST is actually a classification of usnc marine. they are the special forces unit otherwise known as helljumpers.
I've already got some requests for EOD and CQB.
My armor is the same as my live setup.
CQB chest, Security Shoulders, ODST Helmet. in dark grey with red and white trim.
each one serves as a designation for specific spartan functions. EOD - explosive ordinance disposal, EVA - Extra vehicular activity (i think), CQB - close quarters battle, etc. it was more of a way for players to customize themselves without being clones. the elites have similar permutations.
the "fun" (excruciating) part will be figuring out how to wire a pulse configuration into my spartan laser.