Hello ,
The i9-9900KS is a selected 9900k with 5GHZ clock on all cores, but the i9-9900K has 4.7GHZ clock. The i9-9900KF is a 9900k with the graphics unit disabled (you cannot connect the monitor to the motherboard).
The i9-9900KS has not been sold for a while, but pretty much every i9-9900K will do the 5GHZ all core of an i9-9900KS (if that matters to you).
Small summary : It's the same CPU , with slightly more clock and with and without graphics unit .
MFG Junta