Does RTD in plywood mean "Rated" or "Resistance Temperature Detectors"?
The “RTD” in RTD plywood stands for Resistance Temperature Detector.

You have your normal CDX plywood for exterior sheathing.    This plywood is made to function better outdoors - deals with temp changes and moisture better than other forms of plywood.   The main difference between this and your normal plywood is that you will have less delamination.

Now RTD takes the CDX a step further and has controls for the amount of glue used, types of glue, and most importantly making sure the bonding temperature is highly accurate for each layer of veneer.   

This will result in less delamination for CDX and makes it superior for outdoor use.

The grading difference between the two most common types of plywood can be summed up in that CDX is Exposure 1 rated (which means that water hitting it won't hurt it) and RDT is purely Exterior rated (yes I know CDX is still used as exterior sheathing in some places).   Being exterior rated means it will withstand constant exposure to temp changes and moisture.

Depending on the price difference we may buy CDX for subfloors but RDT is used for everything on exterior, including walls and roof.