What is the meaning of VFS
It means "volts full scale".

So your Dataq unit is able to measure voltages between -10 and +10 V.

And if it is, for example, a 12-bit ADC, then each 1-count change in the digital value corresponds to 
$$\frac{20\ V}{2^{12}-1}\approx 0.00488\ V.$$


  is there more to "full-scale" than simply the interval of possible values?


No, not really.