Default application to edit photos: Choose an external editor to launch when using Edit photos. In this example, it is set to Adobe® Lightroom Classic.

Default application to edit movies: same as above but for movie files.

Assign Specific Application to File Type: Use the Set... button. to assign different file formats to different applications. You could, for example, set Lightroom Classic as the default editor but use a different application to open GIFs. These options are shown when using the right-click context menu option on photos in the contact sheet: Edit Photos with.  You can also assign Custom Editors that appear when you select 'Edit photos with..." from the menu.

Maximum Photos to edit at once without a warning: Sending more than this number of images to an external editor will bring up a warning. This warning acts as a safety net in case you send more files than your computer can handle at once. The number of files you can safely send will depend on your system. 

When in RAW+JPEG mode, editing a photo will: When Photo Mechanic is treating a RAW+JPEG pair as a single file, this option sets which one of the pair is sent to an external editor when using the Edit Photos command. You can toggle this preference temporarily by holding down the modifier key when choosing to edit.

Opening Contact Sheet with Navigator or Favorites panes: Sets one of four options for single-clicking or double-clicking in the navigator or favorites panes.

When opening a new Contact Sheet make the Navigator show the path to the folder: When checked, the navigator shows the full path to the newly opened folder.

Photoshop Droplets location: Use this to set where your Droplets are stored so Photo Mechanic can load them into the Send Photos to Droplet menu option.