Ticket fields are custom fields that are typically used to gather more information about your tickets. LiveAgent allow you to create unlimited custom Ticket fields.
Ticket fields Setup
Step 1: Navigate to Configuration>System>Ticket fields
Step 2: Click on Create. Name your new Contact field with a unique name.
Adding Ticket fields in Tickets
Step 1: To add a Ticket field to your Ticket, open a specific Ticket and click on Add field.
Step 2: In the pop-up window click on Add field and choose a field from your options.
Result: New ticket field: Order ID added to ticket