The Metadata (IPTC) Template gives you control over IPTC metadata to efficiently add information to an individual, group, or batch of photos.
Select images in a contact sheet and then open the Metadata (IPTC) Template from the Image menu or by using the keyboard shortcut command-i (macOS) or control-i (Windows).
Type data into any fields you wish to use. You can also use variables or code replacements in these fields. Be sure to select the checkbox next to all fields you wish to change. Other fields will remain unchanged.
Once you are satisfied with your changes, select the Apply Template to Selected button.
By default, the Metadata (IPTC) Template window has every field that Photo Mechanic supports displayed and enabled; however, if you would like to hide some fields so that only the ones that you need are displayed, you can do so in Accessibility preferences.
Pre-Existing Metadata
Check the checkmark next to the + mark if you want to append or add more metadata to a field. Leave it unchecked if you want to delete the old content and replace it with your new entry.
The Description/Caption field uses a dropbox. You can choose to replace the field or place the new content before (prefix) or after (append) the existing metadata. 
Streamlining Your Workflow
If you have a list of frequently used data, such as a list of photographers or subjects, you can use that information to create a custom dropdown list by selecting the dropdown arrow to the right of the field and selecting the Edit... option.
You can also import keywords from lists you have created or purchased.
Using GPS Data
Photo Mechanic can use the GPS metadata stored in your images (whether added by the camera or Imported later) to populate location data. This is called Reverse Geocoding.
Press the globe button next to location-based metadata fields that you would like for Photo Mechanic to reverse geocode. When the globe is green, that indicates that Photo Mechanic will attempt to populate that field with reverse geocoded information. When a globe is selected, reverse geocoded data takes priority over any text you have entered. This means you can safely enter default locations that will only be applied if the image doesn't already have GPS data.