ABCD Defines A,B,C,D as possible keys. If you press a not defined key, you'll hear a beep and will continue as if nothing was pressed. By default, Z is equal to z for choice. Choice shows the possible keys into brackets when is called. 3 and 7 errorlevel 4.
Lets see now what to do with errorlevel.
This evaluates the current errorlevel number. If condition is true the command is executed. To clarify this, read the next example. This is a complete batch program. Lets analyze it.