What is the explicit difference between the fortran intents (in,out,inout)?

Intents are just hints for the compiler, and you can throw that information away and violate it. Intents exists almost entirely to make sure that you only do what you planned to do in a subroutine. A compiler might choose to trust you and optimize something.

This means that intent(in) is not pass by value. You can still overwrite the original value.

program xxxx  
    integer i  
    i = 9  
    call sub(i)  
    print*,i ! will print 7 on all compilers I checked  
end  
subroutine sub(i)  
    integer,intent(in) :: i  
    call sub2(i)  
end  
subroutine sub2(i)  
    implicit none  
    integer i  
    i = 7  ! This works since the "intent" information was lost.  
end

program xxxx  
    integer i  
    i = 9  
    call sub(i)  
end  
subroutine sub(i)  
    integer,intent(out) :: i  
    call sub2(i)  
end  
subroutine sub2(i)  
    implicit none   
    integer i  
    print*,i ! will print 9 on all compilers I checked, even though intent was "out" above.  
end  

• intent(in) - looks like pass by value (and changes of this are not reflected in outside code) but is in fact pass by reference and changing it is prohibited by the compiler. But it can be changed still.
• intent(out) - pass somehow by reference, in fact a return argument
• intent(inout) - pass by reference, normal in/out parameter.

Use intent(out) if is is plain out, to document your design. Do not care for the very little performance gain if any. (The comments suggest there is none as intent(in) is technically also pass by reference.)