Asterisk sip.conf registration question

I am confused about a fundemantal about the sip.com

I want to have people connect to my asterisk server and pass through to another SIP server.

Why is there a register section in the [genral] section of the sip.conf file and also you can have a section to register a peer, which is an external provider. Why both? and which one do I need to use?

Also could someone please clarify if I want each user to authenticate on the other end with the same credentials they use to authenticate with my server, do I need to leave the user name and password out of one section, which section, and put them in the user section? Will this work?

at the top of the forum is a thread called ā€œREAD BEFORE POSTING - Your answer may be hereā€

in there is a link to a free book where you can learn the basics of Asterisk configuration.

I have read the asterisk handbook and it doesnt provide in depth info on this issue.