Equally, the terbri is a list of sumti, as they fill the selbri to produce the bridi. Using the functional definition of bridi,
bridi = x1 (du'u) is a predication with relationship type x2 (ka) produced from sumti x3
You can think of it like this: take the selbri with its ce'u-place "holes" and fill them in order with sumti from the list in the x3. The result is the bridi1, namely a du'u.
e.g. {.i lo du'u mi klama lo zarci cu bridi lo ka ce'u klama ce'u ce'u ce'u ce'u kei mi ce'o lo zarci}
(In practice, ce'u-places can be left out because they're implicit.)
I used the non-logical connective {ce'o} for the terbri, but this can cause problems when the selbri expects a list, which can be produced from {ce'o}. For that purpose, there's an experimental cmavo, {ce'oi}, that exists just for these "argument lists".
As for the actualy difference between {sumti} and {bridi}, sumti1 is a single exact sumti that fills the sumti3-th place of sumti2, whereas bridi3 is a list of sumti that when applied to the function in bridi2 yields the predication bridi1.
.i mi'e la tsani mu'o