DBN is a simple, compact way to represent RNA secondary structure, as matched (),[], {} etc of canonical pairs (Watson-Crick and G--U wobble). The DSSR implementation of DBN follows the convention. DBN extensions that account for non-canonical pairs have been reported in the literature. No standard representation of such extensions exists, as far as I know.
In addition to non-canonical pairs, triplets and quadruplets (or higher, generally termed multiplets in DSSR) also exist in RNA, but they cannot be properly represented by DBN either. I'm generally in favor of keeping DBN simple in DSSR, at least by default. However, I'd consider expanding DBN by additional options, if necessary. I can be easily convinced by seeing concrete/worked examples.
Xiang-Jun