DAC datasheet term confusion: FS-1LSB
"FS" means full-scale, which may be determined by an external voltage. "LSB" means Least Significant Bit - in this case, the voltage represented by one least significant bit. In a 16 bit DAC like yours, that's 1/216 of the full range.

Thus, "+FS-1LSB" means "full scale voltage less one least significant bit". For a full scale voltage of 10 volts, that's 10 volts - (1/216 * 10 volts) = 9.9985 volts.