While looking up Old Chinese reconstructions, I often find square brackets  in the middle of an reconstruction.
For example, Baxter-Sagart system says 寺's old Chinese pronunciation is /*s-[d]əʔ-s/.
I know that // means it's a broad transcription, * indicates it's a reconstruction, and usually  means it's a narrow transcription.
Does [d] in the middle mean that particular transcription is narrow? So for /*s-[d]əʔ-s/, other than [d], all other phoneme are in broad transcription?