You have high and low which you should be able to choose on the unit itself however if its a "coded" system its illegal for anyone but the person or company who owns that code to change, modify, use, etc.... When you buy a coded system from a company they have you hooked for life. They own the code so anything you buy you must buy from them. An "uncoded" system is better because apparatus from different communities can use their equipment in different cities or towns without issue. To better understand coding think of it as a PL tone used in programming radios. Its a sub frequency in addition to the main frequency used. Same applies to preemption only a person owns the PL and you can't use it without them selling it to you.
Hope this helps.