Under the Edit menu there are a number of special commands for selecting files and folders.
Select using filters
The Select and Unselect options show a select/unselect dialog where you select files using filters. You can enter a wildcard filter to select/unselect files, eg. "*.jpg *.tiff" for all jpg and tiff files. Separate multiple filers with a space.
Save/Load Selection to/from File or Memory
Save selection to file will save all the selected files to a filename you specify. You can then load that file and recover all of your selections at any time.
Load selection from file will load a file and treat every line in that file as a selection filter. Each line can be an exact match for a file/folder that you want to select or it can be a selection filter containing wildcards.
Save/Load to/from memory options work the same way but will not survive a restart of Multi Commander.
Compare Folders will compare the paths in the source and target panel and select differences depending on which compare command is called
It is possible to enable or disable whether the compare command should work on only files or only folders.
Compare Folders, Select Newest/Missing
Will select the files that are missing in the other panel or, if folders/files are found in both panels, the newest file/folder is selected
Compare Folders, Select Missing
Will select only the files that are missing from the other panel.
Compare Folders, Select Newest
Will find files/folder that exist in both panels and select only the newest of them.
Compare Folders, Select Duplicates
Will select only files/folders that exist in both panels.
Compare Folders, Select Selected
The files that are currently selected in the source panel will also be selected in the target panel if they exist.