boost takes the volts of an AA or AAA 1.5vdc, and boosts it up to higher voltage (kinda like the portable power units, boosting 4.2vdc to 5vdc for charging USB…same thing but using 1.5vdc, usually pulling more current from the battery that it is delivering
The linear regulator types take the battery voltage and regulate it thru the chips
3A is max power for general purpose, sure you can run MUCH higher currents to a properly heatsinked emitter but stock 3A is about the most you want to push
Li Ion nominal voltage is 3.7vdc, meaning average volts, max charge is 4.2vdc, safe discharge is 2.7vdc, average it out…3.7~, when battery volts get to 3.6 or lower the regulator will still deliver the voltage, it will just be weaker so the light emitted will be weaker, this way a dimming flashlight you know to soon swap out for fresh battery
Alot of reading…but this will help you out
http://flashlightwiki.com/Main_Page