In addition to the variables da, de, and di that we have seen so far, which function as sumti and belong to selma'o KOhA, there are three corresponding variables bu'a, bu'e, and bu'i which function as selbri and belong to selma'o GOhA. These new variables allow existential or universal claims which are about the relationships between objects rather than the objects themselves. We will start with the usual silly examples; the literal translation will represent bu'a, bu'e and bu'i with F, G, and H respectively.

The translations of Example 16.105 show how unidiomatic selbri variables are in English; Lojban sentences like Example 16.105 need to be totally reworded in English. Furthermore, when a selbri variable appears in the prenex, it is necessary to precede it with a quantifier such as su'o; it is ungrammatical to just say bu'a zo'u. This rule is necessary because only sumti can appear in the prenex, and su'o bu'a is technically a sumti – in fact, it is an indefinite description like re nanmu, since bu'a is grammatically equivalent to a brivla like nanmu. However, indefinite descriptions involving the bu'a-series cannot be imported from the prenex.

When the prenex is omitted, the preceding number has to be omitted too:

As a result, if the number before the variable is anything but su'o, the prenex is required:

Example 16.105 and Example 16.106 are almost certainly true: Jim and John might be brothers, or might live in the same city, or at least have the property of being jointly human. Example 16.107 is palpably false, however; if Jim and John were related by every possible relationship, then they would have to be both brothers and father-and-son, which is impossible.