There are three options for controlling slew rate:

1. Pure analogue variation via increment and decrement. RA and DEC can have separate inc and decrement buttons or can be combined on to two buttons.
2. Preset selection via increment and decrement preset. RA and DEC rates are linked.
3. Direct selection via dedicated buttons. RA and DEC rates are linked

If you like to use lots of different slew rates then option 2 is probably the best. If you just use three or four rates then it may be more convenient to use option 3.