There is only one file type that uses the .NUMBERS-TEF extension:
Numbers iCloud Document
Spreadsheet file created by Numbers, Apple's spreadsheet program included in the iWork suite; used as the save format when the user chooses to save the spreadsheet to iCloud.
Even though iCloud uses the NUMBERS-TEF extension, it shouldn't be an issue for users. When you open a spreadsheet on your Mac OS X or iOS version of Numbers, the spreadsheet will still load properly.
NOTE: Numbers spreadsheets saved on the local computer use the .NUMBERS extension.
Updated: May 21, 2015