Bonji is the Japanese word for Siddham characters used in this way, and it names a category of object more than a script. A bonji is a Sanskrit letter or syllable functioning as a sign: on a wooden grave tablet, a stone marker, a printed charm, a temple banner, a bell, a ritual implement, a mandala, or the base of a statue. It is written by people trained to write it, in a calligraphic tradition with its own brush conventions, and it is read by people who mostly cannot read Sanskrit.
That combination is unusual and worth being precise about. The letters are genuine, correctly formed and correctly assigned; the tradition that writes them knows exactly what they say. The audience is not expected to construe them, and the object works whether or not anyone does. What the letter carries in that setting is identification and dedication rather than sentence meaning.
It also means bonji circulate widely outside their tradition, on ornaments, clothing and tattoos, frequently miscopied and frequently misidentified, since a chart of syllables is easy to find and a training in the script is not. A syllable copied wrongly names a different figure or nothing at all. The atlas records what the letters are and what the tradition holds them to be, which is a different thing from recommending that anybody wear one.