Users often enter data in fields that store reference attributes (attributes that reference some configuration objects).
For example, in the Services document they fill the following reference fields: Customer (type: CatalogRef.Customers), Technician (type: CatalogRef.Employees), and so on.
Clicking the selection button in a reference field opens the default choice form of the respective configuration object (Customers catalog, Employees catalog, and more).
You might need to use a custom choice form for a reference attribute. To do so, in the Choice form property of the attribute, specify the custom form. That custom form will be opened every time this attribute is selected in any form.