Linguistically, it has the alphabetical usage in texts for a, or m, or syllabically for am. The "a" is replaceable in word formation by any of the 4 vowels: a, e, i, or u.
Epic of Gilgamesh usage
The am sign usage in the Epic of Gilgamesh is as follows: am-(87 times); AM-(4).[1]
References
^Parpola, 1971. The Standard Babylonian
Epic of Gilgamesh, Sign List, pp. 155-165, no. 170, p. 158, "am".
line drawing, Obverse Amarna letter EA 15-(titled: "Assyria joins the International Scene"). A common
Amarna letter that uses cuneiform am. (Next-to-last cuneiform sign, line 3.)
Right Edge (Obverse) showing am, 3rd line from top. ( am-qut, for "maqātu", "to bow down", etc.; to address someone )