Start From The Consumer and work backwards. First of all define the types of consumers - Joe Public, Businesses, OEM`s (Machines need Software too) Then work out where they might get that software from - Retail Outlet (Define) Online Download (Direct or OnLine Retailers), OEM, Business Specialists etc. Remember that the difference between On Line Retailers and wholesalers has fundamentally changed over the years.
Somewhere in the middle is the Disk Manufacturer or the (Download) Packaging.
Further Back you have the Development, Testing (In House & External - Beta Testing, Public Beta Testing, Release Candidates etc), Further Development and Refinement. Prior to that you`ve got to the Concept itself and the programming.