«بَلْ هُوَ هَيْئَةُ التَّوَصُّلِ إِلَيْهِمَا»
Rather, it is the means by which those two are discovered.
Calling mutaba‘ah, shahid, and i‘tibar three equivalent kinds of narration would confuse results with the procedure that finds them. Mutaba‘ah and shahid describe relationships between transmissions, whereas i‘tibar is the investigative procedure.
Searching the available chains is i‘tibar. If it finds a second pupil of the same teacher, the result may be complete mutaba‘ah; agreement at a higher link may be partial mutaba‘ah; and another Companion's related report may be a shahid. The method is one; the findings differ.