What does this expression mean?

Normal distribution with condition

I am reading a research paper and found the following expression (Eq.28 in the paper below).

It means a Gaussian distribution, but the mean component seems conditional probability-like expression $ \it{\bf{s}}_t | \it{\bf{m}}_{b, t, m}^{(j)}$ . I have never seen this expression before and cannot find any info about it.

The variables $ \it{\bf{s}}_t$ and $ \it{\bf{m}}_{b, t, m}^{(j)}$ are both vectors and $ \bf{\Sigma}_{b}$ is a covariance matrix.

Does anybody have an idea of what this expression means?

Original paper where the expression is.

The original paper can be found here: https://eprints.soton.ac.uk/437941/1/08340823.pdf

Thanks in advance.