Difficulty understanding data over opto-diac
Shorter sentences and more white space, please. 

It's really an opto-triac in the MOC3021 (a low current triac). A triac will break over like a diac, but the voltage is not controlled and it's not usually very low as for a diac. In this case, it's only guaranteed to be greater than 400V (VDRM). 

The current ITSM is peak current, not continuous current. When the opto-triac is used in its typical application it will be driving a much larger triac. When you switch the LED on near the peak of a cycle, the current may only be limited by a single resistor. The triac gate will have very little voltage across it, so the peak current before the triac turns on could be \$\sqrt{2}\times\$ Vrms/R, so for a 240VAC mains, the resistor must be at least 340 ohms to limit the peak current to 1A. 

Power dissipation is another (mostly orthogonal) constraint. Normally the power dissipation due to the opto-triac is not very high since it's only short pulses of (typically) some tens of mA near the zero crossing of each cycle. If you try to control a load directly (which is generally not recommended) the power dissipation spec (including LED power dissipation) may come into play. If the current is 100mA RMS and it drops 2V then the power dissipation will be 200mW plus (say) another 50mW for the LED, then the absolute maximum limit will be exceeded for a temperature of only 43°C, which would normally be unacceptable.