It depends on if your talking about a cable diagram or a schematic diagram. Typically cables are not specified in schematic diagrams because there are connectors on the board. You can find and example of IEC or other standardied symbols here (you can also find examples of the IEEE symbols). You can look at the IEC standard if you want to buy it here. Even building schematics like the NEMA do not provide a cable symbol.

As far as cable diagrams most places do them in house and have there own markings for them. All of the cable diagrams I've encountered usually have a physical representation of the connector and or wire and then call out the material or part number if it is an order able item.

So do what makes sense, anything in a diagram that is atypical should be noted and that is up to the designer. The most important thing for diagrams:

1. All information for the design is Documented
2. That other people understand the diagram

It depends on if your talking about a cable diagram or a schematic diagram. Typically cables are not specified in schematic diagrams because there are connectors on the board. You can find and example of IEC or other standardized symbols here (you can also find examples of the IEEE symbols). You can look at the IEC standard if you want to buy it here. Even building schematics like the NEMA do not provide a cable symbol.

As far as cable diagrams most places do them in house and have there own markings for them. All of the cable diagrams I've encountered usually have a physical representation of the connector and or wire and then call out the material or part number if it is an order able item.

So do what makes sense, anything in a diagram that is atypical should be noted and that is up to the designer. The most important thing for diagrams:

1. All information for the design is Documented
2. That other people understand the diagram