A number of Tibeto-Burman languages exhibit morphological ergative alignment, while others clearly do not. In these languages, matters of information structure determine core argument marking. Specifically, both A and S marking may be used to indicate topic, contrastive topic, broad focus, and/or contrastive focus. It is most often A or S, not P, that is assigned such status and between A and S, it is most often A that takes marking. Preference for topic or focus marking on A creates the impression of ergative alignment, but an ergative alignment analysis is untenable as S may be marked under the same conditions and with the same morpheme as A. Considerations of discourse-level clause interpretation in Tibetan, Meitei, and Burmese show that information structure not transitivity determines A and S marking. The presence or absence of marking based on information structure is characterized as “unique differential marking”, distinguishing it from the differential marking observed in ergative and accusative alignment systems.