It could be the letter understood either graphically or audibly, and the variety of ways in which one can choose a basic unit in the study of linguistics means that you need a special word for that unit, which is characteristically "the tagmeme."

它既可以以图像的方式理解，也可以以声音的方式，因为每个人研究时选取的,基本的语言学单位不一样,所以需要一个特殊的名词来指代这个单位，这就是，法位“

耶鲁公开课 - 文学理论导论课程节选