x1 is a defined Lojban word, considered as a word as per Lojban's grammar, with meaning/role x2
In Lojban, particles such as "lo", "ku" etc. are considered to be words, whereas they may not be considered so according to other standards (for example, as "lo" and "ku" cannot appear in isolation, they are arguably closer to clitics than independent words). See also jbokravla (native Lojban word) and valsi.