what difference between individual module and 2 module in one module

what difference between individual module and 2 module in one module

i mean module allow to you create 2 or 3 or 4 module in one module
module m1
''''''''''''''''''''''''''
end module

module m2
''''''''''''''''''''''''''''
end module


or working with one module

module onemodule
end module

thanks for understanding myquestion