For this milestone you have to do a proper market research:
Determine the size of your target market (including the different sub-markets)
Determine the approximate number of players / competitors in your targeted market
Present the competition and if possible market share for each competitor
Estimate your potential market share in year 1, 2, 3, 4 and 5 after launch
Determine the total market value and the value of your market share for years 1, 2, 3, 4, 5
At the end of this milestone you should be able to draw a conclusion regarding the profitability of your business.