There are different Usages of the letter B.
1. the second letter of the English alphabet
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
This letter denotes the second in a set of items, categories, sizes, etc. e.g. "B batch of medicines is created after the A batch and so on." It also denotes the second of two or more hypothetical people or things. e.g. "A person is sitting facing towards North and the B person is sitting facing towards South." It also denotes the second highest class of academic mark. e.g. "Rahul gets B grade marks." Further, it denotes the human blood type (in the ABO Blood Group System) containing the B antigen and lacking the A Antigen. In the UK, it also denotes the secondary category of road. e.g. "the B4248" It also depicts the second highest-earning socio-economic category for marketing purposes, after top management and senior professional personnel. Numbers printed on right, it denotes a series of international standard paper sizes each twice the area of the next, as B0, B1, B2, B3, B4, etc.
2. Music & Chess
In Music it denotes the seventh note of the diatonic scale of C major. And, in Chess, it denotes the second file from the left, as viewed from White's side of the board. B is also used for Bishop in Chess while recording moves. e.g. "Be6"
It is used to denote the grading of lead pencils where B stands for Black. e.g. "2B, HB" Besides, B letter of English Alphabet is also used in Belgium as International Vehicle Registration. B in its lower case as b is also used in Cricket scorecards where it stands for Bowled. e.g. Sachin Tendulkar b Shane Warne 99."
B is a symbol of Element Boron and in Physics B = Magnetic Flux Density. Besides, in Particle Physics b=barn that gives a unit area of 10−28 square metres.
Plan B which represents an alternative strategy. e.g. "There must always a Plan-B to succeed in Business."
The most Common Noun which is used to learn this letter at the Nursery level is Boy; B for Boy.