If checked, a single click is all that is necessary in the Drives and Directories window (and the DirMarks window) to read or refresh that drive or directory. An double click has no consequence or effect. To select a drive or directory without causing a rescan, select with the LMB and the Ctrl key down or select with the RMB. The latter also causes the context menu to appear.
If unchecked, standard OS/2 single and double click moding is in effect.