1.1 Content characters
The content of a Stan program must be coded in ASCII. All identifiers must consist of only ASCII alpha-numeric characters and the underscore character. All arithmetic operators and punctuation must be coded in ASCII.
Compatibility with Latin-1 and UTF-8
The UTF-8 encoding of Unicode and the Latin-1 (ISO-8859-1) encoding share the first 128 code points with ASCII and thus cannot be distinguished from ASCII. That means you can set editors, etc., to use UTF-8 or Latin-1 (or the other Latin-n variants) without worrying that the content of a Stan program will be destroyed.