The User / Client Variables dialog is useful when working on different jobs and with different clients when you wish to make sure images are easily trackable. By defining these variables you can use any of this data in the same way as normal variables. Think of User Variables as user-defined text strings that save having to repeatedly enter identical info about yourself.
If you have a group of regular clients you could fill in this form for each one, using the Snapshot button to save each set of data. Each time you do a job for a client you can reload the form in two clicks by selecting the Snapshot button again and selecting the client from the list. This will populate all the fields as you last saved them. The User/Client dialog stores/loads the Job name with the Client snapshots.
The User/Client window checks for changes on Cancel and warns on changes.
With the fields filled out, you can now use the associated variables to, for instance, create folder names based on the client's company name or more powerfully, to add data into the Metadata Template. Inserting {clientname} {clientcompany} and {clientphone} into, say, the Special Instructions field will tag all the files with your client's name, company and phone number.
You can get to the User/Client Variables window by navigating to Edit>Settings>Set User/Client Variables... , or selecting the Job... button in the Ingest window, the Metadata (IPTC) Template window, and the Metadata (IPTC) Info window.

Once defined, User/Client variables behave like other Photo Mechanic variables. Fill out as many or as few fields as you need. The first section applies to anything you want to label as “client.” The second for anything you want to label as “user.” Each field corresponds to a single variable, such as {username} or {clientaddress}. For Photo Mechanic users on macOS can hover the mouse cursor over a textbox to see a tooltip containing the variable associated with the field. User/Client Variables are found at the bottom of the Variables list that is accessible via the Variables... button in various windows that contain user-editable textboxes such as the Ingest, Metadata Template, and Metadata Info windows and some of the Settings/Preferences.
