This information comes from the Robocopy.exe documentation PDF file for Windows XP version, but it’s the best description I’ve been able to find. From page 15 of that document:
Using Robocopy File Classes
For each directory processed, Robocopy constructs a list of files in both the source
and destination directories. This list matches the files specified on the command line
Robocopy then cross-references the lists, determining where files exist and comparing
file times and sizes. The program places each selected file in one of the following
|File Class||In source||In destination||Source/Dest file times||Source/dest file sizes||Source/dest attributes|
|Newer||Yes||Yes||Source > Destination||n/a||n/a|
|Older||Yes||Yes||Source < Destination||n/a||n/a|
|Mismatched||Yes (file)||Yes (directory)||n/a||n/a||n/a|
By default, Changed, Newer, and Older files are candidates for copying (subject to
further filtering, as described later). Same files are not copied. Extra and Mismatched
files and directories are only reported in the output log.
Normally, Tweaked files are neither identified nor copied – they are usually identified
as Same files by default. Only when /IT is used will the distinction between Same and
Tweaked files be made, and only then will Tweaked files be copied.