Similar to [an earlier question][1], I am now looking at S&P's sovereign ratings. Here, as in the case of Moody's, a few things are unclear to me in terms of the definitions used by Standard & Poor's.

1. Several sovereign ratings (Foreign Currency LT Debt, obtained through ``CSDR`` in Bloomberg Terminal), contain a suffix ``u``. For the case of Belgium, example given, ratings are denoted as ``AAu`` or ``AA+u``. What does the ``u`` stand for? 

2. What does the rating ``SD`` stand for?

[1]: https://quant.stackexchange.com/questions/739/are-public-historical-time-series-available-for-ratings-of-sovereign-debt]