What do you understand by PUBLIC assembler directive?  

The PUBLIC and EXTRN directives are very significant to modular programming. PUBLIC used to state that labels of data, code or entire segments are accessible to other program modules. External or EXTRN declares that labels are external to modules. Without those statements, modules could not be linked mutually to create a program using modular programming techniques. These might link, but one module would not be capable to communicate to the other.

The PUBLIC directive is placed into the opcode field of an assembly language statement to describe a label as public; therefore the label can be used by other modules.