What are the these commands and the differences between them? Why are these two mandatory? R1(config-telephony)#auto assign 4 to 6 #Automatically assigning ext numbers to buttons# R1(config-telephony)#auto assign 1 to 5 #Automatically assigning ext numbers to buttons#

In packet tracer I am trying to register 2 IP Phones on a CME Router, but I am not able to understand if following commands are mandatory for registeration or not. Only after giving these commands the phones are getting registerd and after that even if I do "no" for these commands then also the phones remain registered. 

RouterA(config-telephony)#auto assign 4 to 6 #Automatically assigning ext numbers to buttons#

RouterA(config-telephony)#auto assign 1 to 5 #Automatically assigning ext numbers to buttons#