What does maximum current density mean?
The maximum current density is the maximum current allowed through the digipot.
In potentiometer mode, this isn’t an issue as the resistor is quite high and
this limits the current. The specification needs to be taken into account when
the part is used in rheostat mode and the resistance can be small, e.g. 500
ohms, maximum voltage in the rheostat terminals is 500 * 6mA = 3V.