IMPERFECT: Ich habe das Mädchen, das auf dem Sofa saß, gesehen. PERFECT: Ich habe das Mädchen, das auf dem Sofa gesessen hat, gesehen. Both ways are correct. Also, both your seemingly contradictory pieces of information are correct. :-) In many situations modern German doesn't distinguish between perfect and imperfect, so you can use either. But if it does make a difference if the action is ongoing or completed, you use the appropriate one. So in your example, the second version would emphasize that the action consisting of "sitting" is finished; in other words, she doesn't sit anymore on the sofa. But since the whole action happens in the past anyway, that may or may not be relevant, depending on the context. As a rule of thumb, the distinction becomes important if there are two actions in one sentence and the first influences the second. For example, in sentences with "nachdem" one would normally use the perfect for the first action, because the first action will have to be finished before you can do the second action. And so on. I hope that helps at least with some of the confusion.

Ich habe das Mädchen gesehen, das auf dem Sofa saß -this form is rather uncommon, in believe due to need to do the flexion, saß- Ich habe das Mädchen gesehen, das auf dem Sofa gesessen hat -i would say it like this, it´s easier to think and say and has the same meaning-

Present perfect: I saw the girl who has been sitting on the sofa. Past Perfect: I saw the girl who had been sitting on the sofa. Future Perfect: I saw the girl who will have been sitting on the sofa.