My way of viewing this is that :proc_lib is a tool which is used for building OTP compliant servers. It is used for building the standard OTP behaviours like :gen_server/GenServer but you can of course use it for building your own OTP compliant processes, called in the OTP docs for special processes.
Why would you want to build your own processes and not just use the standard provided ones? Well maybe you need something which is completely OTP compliant so it fits in and can be run by supervisors but just doesn’t really fit to be built using the the standard behaviours, e.g. maybe you want much better control over message handling than you can get with a GenServer.
So building your own special processes is actually not that difficult. There are 5 rules to follow and the first one is to start it using :proc_lib. You then need to detect and handle system messages in the right way but most of that work is done for you with the :sys module which you call in the right places. Finally you need to terminate the process at the right time in the right way. Quite simple really.
All this is an hour or so in our (Erlang Solutions) Erlang OTP course. 
BTW the :sys module contains a lot of useful functions for managing OTP behaviours.
