There is the trite point that dividends are paid out of cash rather than profit and it is usually the availability of the former which constrains what can be distributed.
However, the logic of the free cash flow to equity model and the use of free cash flow after net reinvestment as a proxy for the distributable element of return to investors, is based upon the original concept of income first proposed by Hicks that what is distributable is what can be taken out and leave the value of the business unimpaired.
Value in this sense is taken to be the present value of the future cash flows of the business so what can be distributed is the change in the present value of future cash flows.
The free cash flow to equity model in valuation works on the same principle as the dividend valuation model in that we seek to determine the free cash flow, after net reinvestment sufficient to maintain the operating capacity of the business, which could be available for distribution to the equity investors.
The legal definition of what may be distributed is almost always considerably more than what should be paid out in cash if the business is to maintain the operating capacity and hence the value of the business.