Final answer:
To have a CI class appear in the Configuration item look-up on a change form in the CI Class Manager, the class must be set as a Display Class.
Step-by-step explanation:
To ensure that a CI (Configuration Item) class appears in the Configuration item field look-up within a change form on the CI Class Manager, you need to set it as a Display Class. The 'Display Class' determines whether the class will be visible in look-up dialogs when users are associating CIs to other records, such as incidents or changes. On the other hand, the 'Principal Class' usually refers to the overarching class under which various subclasses may be organized, the 'Active Class' typically indicates classes that are actively used in the configuration management database (CMDB), and the 'Configuration Class' generally refers to all classes used for configuration management. However, it is the Display Class designation specifically that controls visibility in the field look-up.