not understanding the part of the answer for drawn turing machine

Question

Could someone please tell me what does capital B mean here ? of course I know R and L stands for right and left...

and also I know for example if we have a,b,R (which tells you if you have an a , make it into b and go right), so my question is why is capital B here and what does it mean ?

Answer 1

In his book Introduction to the Theory of Computation, Michael Sipser uses $B$ to stand for "blank" on the tape. (Confirming DKal's comment)

I have also seen this usage in a book by Sudakamp on formal CS.

So the transition from $q_1$ to $q_2$ in your machine above prints a $b$ to the first tape and "stays," then prints a blank to the second tape and moves that tape left.