There is a really simple way to break down the difference between RPK and Non-RPK
Friendlies
Nearly all guilds have a list of friendlies. Could include many guilds, it could be very short or potentially only include their own guild's alts. Killing friendlies is almost never allowed in a functional guild.
Hostiles
Nearly all guilds have a list of hostiles. People who are against them, have slighted them, they're being paid to kill etc. Most guilds strongly encourage the killing of hostiles.
Neutrals
Neutrals are the guilds/players you have not real relations toward friendly or ill. Perhaps you don't know them, perhaps you just have no strong feelings about them.
RPK Guilds - Can kill these neutral players because they are not friendly. "Not Blue, Shoot It" is the mantra of the RPK mindset.
Non-RPK Guilds - Are told by their leadership not to kill neutral players without good cause. "Not Red, Don't Shoot" is the mantra of the Non-RPK mindset.
ARPK Guilds - Are Non-RPK guilds but beyond that, as "Anti-Random Player Killer" implies, they are on a crusade to combat RPK guilds and players.
There is a meaningful distinction between the groups. IMO all are needed to make a healthy PvP-MMO.
"What about RPK in some places, not RPK in others." It depends on the context. When our guild was allowed to kill neutrals everywhere but Fab and Kranesh we called ourselves RPK as killing neutrals across nearly the whole map is pretty random, we were just nice in one area. Now that we are only specifically allowed to kill neutrals without provocation in the town of GK (which our alliance is trying to control), we're non-RPK as locking down a single specific city for strategic reasons is hardly random. Kind of just have to use common sense and not make it more complicated than it is.