Class F common stock is a special class giving founders more powerful voting rights, often some multiple of ordinary shareholders' rights. Class FF stock is a series that allows founders to convert their shares into other equity tranches as they raise further rounds of funding. I have read that the founders of well-known tech companies owned these types of shares while they raised capital at various stages. Founders' shares allow the early team to retain an extraordinary amount of control even after outside investors buy a majority of their company's common shares.
The hottest startups have the luxury of dictating terms for capital raising. Venture investors eager to raise the valuation multiple of the next sure thing may be willing to ignore the disadvantages they will face dealing with founders owning these special share series. Late-stage investors may not care about founders retaining control this way if they can jump on the gravy train of a pending IPO at the eleventh hour.
I am not a securities attorney. There is no way that anything I mentioned here could ever constitute legal advice. I mention these things because founders will eventually encounter investors who may wish to discuss these stock classes as part of a term sheet. Startup founders should consult their attorney before deciding whether issuing these types of shares makes sense.