Maximum Clock Input Frequency Datasheet Confusion
They are expressing that the maximum clock frequency is a function of VCC and temperature. At 2V you won't get parts that max out below 6 MHz, but they could work higher, 17 MHz is typical, and they don't spec a max which means if that chip "works" at 18 MHz then great, but they won't guarantee it.  So at 2V your max clock that you can design for is 6MHz. 

All that is at 25C, if you need harsher temperatures, then use the appropriate temperature column.