In Circuit What is R ,Y ,B written in box?
In the country where this circuit was published, you will almost certainly find the colour code for 3-phase wiring uses Red, Yellow and Blue wires for the three phases.

Searching "Three phase color codes" leads to this page which shows Red, Yellow and Blue were used in the UK (before 2006), Europe (before 2004) and India, so you are either looking at a fairly old European circuit or an Indian one.