It really doesn't mean anything as far as operational functions. It just identifies it as SwimPure vs AquaRite.
It is the name sent to an automation system.
AL-0 is AquaRite
AL-4 This changes the default display from average salt to percent output.
AL-5 This is used with Jandy automation.
Aquarite, naturesoft, mineralsprings and smartpure are essentially the same unit with different branding. Hayward does private labels for certain companies. al-0 to al-3 are basically equivalent other than a superficial name change. AL-4 is not about the name, it just changes the default display. AL-5 needs to be selected for the Jandy automation to control the unit properly.
Also note that the Swimpure is the same as an aquarite. Hayward makes the same model under a bunch of different names. Swimpure, H40, mineral springs, naturesoft, smartpure etc.