Block Started by Symbol - This refers to the way uninitialized global variables are handled by the compiler. The BSS section does not contain any data, the START and END addresses are simply maintained so that the RAM area may be zeroed out efficiently at run-time. The BSS section does not appear in the binary image of the application.
unsigned char var; // 8 bit variable uninitialized variable allocated into the .bss section.
unsigned char var2 = 25; // 8 bit initialized variable allocated into the .data section.