Regexten and regcontext

I am searching and looking for information regarding regexten and regcontext. I can successfully use it to register create the Noop statement, but I’m and unclear on how this is useful. Does anyone have an links to documentation and/or use cases/examples for regexten that I can read?

[ Context ‘sip_autoreg’ created by ‘pbx_config’ ]
‘1700’ => 1. Noop(desk1) [SIP]
‘1701’ => 1. Noop(rick) [SIP]
’_XXXX’ => 2. Dial(SIP/${EXTEN}) [pbx_config]
3. Hangup() [pbx_config]

-= 3 extensions (4 priorities) in 1 context. =-