How to use alternate flush mode on toilet
It's not an alternate mode, per se. Those values are the minimum and maximum tank volumes based on float setting. [Correction: These values seem to apply to High Efficiency (HE) and Ultra-high Efficiency (UHE) bowl/valve options. Since the values are printed on the bowl, they don't tell you much about the particular tank installed. However, the stamp in your bowl does indicate which you have (1.6 gpf).]
Your toilet is no different from all those that came before in the sense that when you lift the flappy-doodle with the levery-bit all the water falls out. They're rather unintelligent devices.
To reduce flush volume, adjust the float to a lower level. There's probably an adjustment screw on it where you can move it down. This shuts off the water sooner on fill.