r1 is a predictable/regular affix for word/concept r2, with form/properties r3, in language r4.
Literally 'rule affix'. In Lojban, jvarafsi are the familiar four-letter (or, more precisely, four-segment) rafsi, which are fully predictable in form, and of which each gismu has only one. For example, -badn- is the jvarafsi of badna 'banana', and can belong to no other word. The form of the affix -badn- may be inferred from the form of the word badna, and vice versa (if the final vowel of badna is known).