x1 (quoted text) is a “bu letteral” of character x2 (zoi quote).
This term is specific to Lojban. A “bu letteral” is defined as one word which has been made to a letteral with “bu”. x1 is a quote (eg. with lu…li'u) containing the “bu letteral” (including “bu”), x2 quotes the character which the “bu letteral” stands for verbatim, which requires to use a zoi quote. See also: lerfu.