Just want to clarify what is spam again. Having links in your signature that promotes or advertise something is counted as spam but having your own blog in your link is now, right? So, how about having SEO links not affiliate to the poster? Does that count as spam too

Links in your signature do not by themselves count as spam. It's only if you post with the sole intention of promoting your signature that they count as spam. This is a difficult pattern to nail down accurately, so if you have links in your signature, you might find otherwise innocent posts deleted as spam.

So if a poster have a link in his signature and the link is about laptop repair but he post something useful, that is not counted as spam. But if he posted something useless to the thread, that count as spam. Correct?

Links in signature are always allowed unless they contain foul language, are illegal, etc. Only the actual post content qualifies for having moderator action taken upon it (including being called spam). However, as Julienne pointed out, actual post content can qualify as being spam if it is nonsensical, etc.

So if a poster have a link in his signature and the link is about laptop repair but he post something useful, that is not counted as spam. But if he posted something useless to the thread, that count as spam. Correct?

Yes. Although, the definition of "useless" can be very subjective, so it really needs to be very obvious that it was only posted to promote the signature.

Ok, thanks for the clarification:)