I suspect "1st" means first offense, and that the prosecutor charges all of the BAC levels so that if the defense successfully challenges a particular level, all of the lower levels remain (sort of like a lesser included offense). Is this correct?
Example charges: DUI, 1st DUI, BAC .08 or more 1st DUI extreme, BAC .15 to .19 1st DUI extreme, BAC .20 or more 1st.
Criminal Defense Attorney
The penalities for DUI's can be made more harsh if the person convicted has prior DUI convictions within a certain amount of time. And you are correct in regards to why all are charged. Plus, for the DUI/Impaired, if the prosecutor can prove the defendant's BAC was above .08%, then the jury can assume the driver/defendant was impaired.