Diodes are polarized components that must be placed on a PCB in a certain way. However, a board that has a (+) silk screen marking is not enough information to tell the assembly house how to correctly insert a diode because there are different types of diodes. 

For zener or barrier diodes, the (+) marking may or may not be the cathode of the component. Furthermore, an LED usually has a (+) anode, but not all the time.

To verify the proper way to place a diode onto a PCB, it is best to use a symbol of a diode or an indication of an anode or a cathode as per standards of the electronics industry. 

You can also use an “A” or a “C.” Ensure you also mark the PCB with a reference identifier such as D3, D4, etc. so the assemblers know it isn’t a capacitor. By making your silk screen marks clear, the PCB can be assembled on time and at the lowest cost.