I was going to say something about reading the instructions but then I went to the S&W site and looked at the Owner's Manual. It is generic for all models and calibers of recent production revolvers and the material in it about standard and +P ammunition is confusing to ME and I have been doing this stuff for 50+ years.
At one time you would have gotten a leaflet that listed suitable ammunition for that model. It would have said something like ".38 S&W Special also accepting .38 Short Colt, .38 Long Colt, .38 Special Midrange, .38 Special, and .38-44 High Velocity*"
*.38-44 High Velocity was the equivalent of +P+ and would have been recommended only for the big N frame .38s and .357 Magnums.
SPL is the abbreviation for Special. CTG is the abbreviation for Cartridge.
The all caps marking seems to confuse the novice gunowner, but that is all it is, no secret code; although I have been tempted to make something up.
(If you see an older gun marked, say, .32 S&W CTGE, it is not a Smith and Wesson, it is a minor make like Harrington and Richardson or Iver Johnson in the .32 S&W CALIBER. Smith and Wesson do not, did not use CTGE as an abbreviation for Cartridge.)