How exactly are WHM accounts structured?

For example:

reseller.com <- whm account 1GB TOTAL, 500MB website
client1.com <- sub-account 250MB
client2.com <- sub-account 250MB

or

reseller.net <- whm account 1GB TOTAL http pointer to reseller.com website
reseller.com <- sub-account 500MB reseller website
client1.com <- sub-account 250MB
client2.com <- sub-account 250MB

Say all 3 .com domains have a site and all require a cpanel login. Should all 3 show up under the reseller WHM account to be able to create cpanel logins or is the reseller domain usually excluded? How is the reseller quotas usually handled so that the reseller can have a cpanel login and seperate webspace from the client accounts? Would a reseller login need to be created by itself and the reseller create its own cpanel login similar to a client but with a different login from the reseller login? For example, instead of logging in as reseller, maybe they login as resellermaster and just not have any website space setup for resellermaster? Maybe a second domain like reseller.net for resellermaster to point to reseller.com cpanel space?