4. No referral code (ref link) spam. [1]
If you are posting a reply on a thread, and your answer is relevant. Would adding your referral link instead of the plain url be a problem?
For example, you are suggesting service a-dot-com as an answer's to OPs question. Would a ref link (a-dot-com?ref=my-ref) be allowed?
If the post is directly on-topic (like in your example) and is constructive (i.e. not just parotting content that was already posted), it's allowed.
Also, it would be neater if you enclose your referral link in
[url=http://(ref link)](then type the actual link)[/url]
which will make the thread flow much better since there is a tendency that the topic will direct to...
another concern.