Res writes a plain r, and its cognates are the Hebrew resh, the Syriac rish and the Arabic ra. The Northwest Semitic name means head, and it is one of the acrophonic names the Canaanite alphabet is built from, the sign being named for a word that begins with its sound.
What makes this letter worth a second look is that the sense has not been lost on the Ge'ez side. Ge'ez has the word re'es for a head, from the same Semitic root, and it is a live word, used for a head, a top, a chief and the first item of a series. The letter name res and that word are recognisably the same thing. For most letters of the fidel no such statement can be made, because the names have worn away from any word still in the language, but here the connection is visible without reconstruction.
That is worth saying precisely because it cannot be generalised. Two or three Ge'ez letter names still show their word, with bet and gaml the other clear cases. The rest do not, and a chart that assigns a meaning to all twenty-six has filled the gaps in from Hebrew. The atlas leaves them empty and says why, which is less satisfying to read and is the only honest option available.
That is also why the atlas records a meaning against three of these twenty-six rows and leaves the rest blank rather than filling them evenly. An uneven record that matches the evidence is more useful than a complete one that does not, and a reader can see at a glance which letters have a name that still means something and which have only a name.