Prior to 12c, PMON process used to handle the instance registration with listner, From oracle 12c, a new background process called lreg takes care of this instance registration, Which reduces the load from pmon process.
$ ps -ef | grep lreg
oracle 3536 105780 0 17:21 pts/5 00:00:00 grep lreg
oracle 95761 1 0 Oct26 ? 00:01:14 ora_lreg_ORCL -- >>>>> lreg for oracle db
oracle 116911 1 0 Aug23 ? 00:04:24 asm_lreg_+ASM --->>>>>> lreg for ASM instance
Basically, LREG process does below things.
1. Registers instance information with the listener.
2. It is a critical background process for each database instance (if it is killed, Oracle goes down).
3. It does everything PMON used to do and report: service_update, service_register, service_died in the listener.log.