It is an attribute. (Luma, 441; Istilāh, 536)
According to the people of Sufism (ahl al-taṣawwuf), vesm and resm are two attributes that occur in pre-eternity (azal) just as they occur in post-eternity (abad). Here, by "pre-eternity" (azal), what is meant is the knowledge of Allah, not the state of those two attributes as mujarrad (abstract) in pre-eternity. These two attributes are indicated in this way. Vesm means trace, sign. In this sense, it refers to a divine sign that appears upon or within the servant, indicating that he is among the people of arrival (wuṣūl) and realization (taḥqīq).
As for resm, it is the manifest appearance of the Real (al-Ḥaqq) upon the servant. This occurs during the return from the ḥāl (spiritual state) he claimed to possess. The manifest sign that appears upon him confirms his claim. (Futūḥāt, II, 497)
Resm is His creation and attributes, for rusūm (forms, shapes) are traces. Everything other than Allah is a trace of His acts. Therefore, those who say that resm is an attribute that occurs in post-eternity just as it did in pre-eternity mean that creatures and all their attributes exist by the power of Allah. (Kāshānī, 150; Jāmiʿ, 84)