I've received a reply from Support, and it's clear enough. There are three ways to refer others:
A) the rewards.cgi script
If somebody clicks such a rewards link, they get a cookie. If they already have a cookie, the older cookie wins. If they then sign up (and don't enter any referral info), then I get the reward such as I have chosen: either 10% forever or $97 once.
B) User enters my email address as referral
This overrides option A.
If somebody signs up and enter my account's primary email address, then I get the reward such as I have chosen: either 10% forever or $97 once.
C) promo code
This overrides options A and B.
If somebody signs up and enter my promo code, then I get the $97 once, minus any rebate that is given to the new user. No "10% forever" is given when a promo code is used.
I am going to publish this in the wiki.
[color=#CC0000]Get $50 off[/color] with promo code "LESS"! See here what else you get!