a question: task, function vs module

Thanks for all that helped in my last problem about det FSM. As a beginner, I always meet this situation: When some statements that perform a specific function and would be instantiated more than one times in a module, we may instantiate the statements as a function, a task or another module. For example, I may write a function, a task or a module to form a function of compare two signed number, and then instantiate it in my module. I want to ask that after synthesis and fitting, where are the differences when I am respectively writing statements as follow: /////////////////////////////////////////////////////////////////////////////////////////// //situation I module top_module(...); ... sub_module u1_submodule ( .port1(wire1), ... ); endmodule //sub module module sub_module(...); ... endmodule ///////////////////////////////////////////////////////////////////////////////////////////// //situation II module top_module(...); ... task_name(...); ... //task declaration task task_name; ... endtask endmodule ////////////////////////////////////////////////////////////////////////////////////////////// //situation III module top_module(...); ... function_name(...); ... //function declaration function function_name; ... endfunction endmodule ///////////////////////////////////////////////////////////////////////////////////////////////// Thanks a lot for any reply about differences between the 3 situations. YuQing Youth