x1 (ka) is the relation of predicate x2 (du'u), which has arguments x3 (sequence of sumti).
In Lojban, for example, "lo ka ce'u prami ce'u cu selbri lo du'u mi prami do ku mi ce'o do". In the mathematical notation P(x, y, z) for a predicate, the relation P is the selbri. Note that x1 is not textual, even though Lojbanists informally tend to call a word or phrase "the selbri" of an utterance. For this usage see selbrisni.