- The symbol is for a lamp (with rays of light eminating from it).
- The 'B' is, most likely, for 'blue'. This would be typical for an industrial machine reset button.
- The dotted line would usually be connected to a switch contact indicating that the lamp is part of an illuminated button.
I can't find a reference for the symbol but it's quite common on American machine ladder schematics that I have seen.
Figure 1. Indicator lamps on a ladder diagram. Image source: All About Circuits.