What is the"keep high efficiency if available" option in the photos tab in icloud for windows?

Hi flboy44,

Thanks for coming to Apple Support Communities for help with iCloud For Windows and the "Keep High Efficiency If Available" option.

Have a look at this page: Turn iCloud Photos on or off on your devices.

"Keep high efficiency original if available: By default, any High Efficiency Image File Format (HEIF) photos are converted to JPEG format before they are downloaded to your PC. If you select this option, both the HEIF and JPEG versions are downloaded."

Have a fantastic day!