Forbidden lines of high ionisation stages (e.g. [FeIV], [FeX],[ FeXI], [FeXIV]) are an important tool to study the stratification (ionisation potential - line width) in line-emitting regions of AGN. They also provide an opportunity to study X-ray absorbing plasma at high spectroscopic resolution.
Papers:
Wagner, S.J.: Coronal Lines from BLR, NLR and ENLR, in: IAU Colloquium 159: Emission Lines in Activ Galaxies: New Methode and Techniques, B. Peterson, F.-Z. Cheng and A.S. Wilson (Hersg.), PASP Conf. Ser., Vol. 113, S. 298, 1996