Base on my experiences, there were 3 different types of friends. 1. Hi-Bye friends. They think that it is redundancy to talk to u more than 2 sentences. ( 1st is Hi, 2nd is Bye ). 2. Good Friends, we all had a lot same topics. We were all quite matched just like smoothly running gears. 3. Close Friends, you are willing to show your care to each other. The obvious indicator is you can express your weakness to him/her and he/she did not take this advantage but care about this.

I just think that those people did not need to categorize their friends. They have just already known what they should belong for their friends. Their friends just categorized themselves for him/her.