apk+ extension and compatibility

On an Android TV, I made some backups with CX File Explorer. When I tried to install it on a different SmartTV, the format was not recognized and I realized about a "+" symbol after the "apk" string. I renamed the apk+ to apk, but Android didn't recognize the installation package. So, I installed CX File Explorer, and using it, the apk showed the proper app icon, and installation was successful. What is the apk+ extension? Is it a new "apk" package version? Just a "proprietary" format with a misleading extension name? Are there more apps recognizing these packages?