Hi! This is just an approximation and not an exact science. The cash flow is seen as a result of earnings power and rational investment strategy for each of the quadrants. For Cash Cows and Stars the rationale is as follows:

• Cash Cows: Strong relative market share in a mature low growth market --> low investment (outflows) and high earnings (inflows) --> high positive cash flow
• Stars: Strong relative market share in a growing market --> high investment (outflows) and high earnings (inflows) --> neutral cash flow (since earnings and investments cancel each other out)

Hope this helps!

Cheers, Sidi