An RTD is a resistance temperature detector. It is essentially a thermocouple, designed to provide real time, very accurate temperature readings. RTD plywood appears to be exposure grade 1 plywood manufactured using RTD technology to control the temperature of the glue during application and curing. Presumably this makes the plywood superior to CDX plywood, which by the way stands for C face on one side, D face on the other side, exposure grade 1.
CDX is NOT exterior grade plywood. Exterior grade plywood uses superior glues to CDX. Since CDX is exposure grade 1, it is suitable for use as a substrate in damp environments, such as sheathing for roofing, or underlayment for tile. RTD plywood appears to refer to the manufacturing process; the only way to know the exposure class on RTD plywood would be to check the rating on the sheet, it should be stamped on the sheet.