Symptoms

The user has accidentally closed the Select Document Definition Panel on the left side in Verification and cannot get it back. There is no Side Panels menu option in the View menu. 

Cause

This is a known issue that was fixed in FlexiCapture 12 Release 3 Update 2 (build 12.0.3.4038). All versions above 12.0.3.4038 have the Side Panels menu option in the View menu.

Resolution

If the FlexiCapture update is not applicable, the possible workaround is to change the registry key 
HKEY_CURRENT_USER\Software\ABBYY\FlexiCapture\12.0\Shell\Frames\ShowDocumentDefinitionPane value to true.