We see thes typically in Japanese bearing catalogues. As far as i am aware these are specified by the Japanese Industry Standards (JIS).
e.g. a shaft fit may be recommended to be either j6 or js6 - both being acceptable to the bearing manufacturer.
E.g. for a 50mm shaft, j6 would be -5/+11, whilst js6 would be +/- 8. The tolerance band is the same, but the base level changes slightly.
Lester Milton
NBC Group Ltd, Telford, Shropshire, UK