Custom forms (categories) contain custom fields (parameters). Categories and Parameters are what you will find in the API. DE: stands for Data Element and is the shortcut to refer to custom fields on a data object such as User, Project, Task, etc. If you set a custom field via a report in the UI it auto-attaches the first custom form that it finds with that field. If you set a custom field via the API and no custom forms are attached with that field, the value isn't saved at all. Category (custom form) used to be limited to one per object, but you may now have multiple forms on an object. But no matter how many times a field appears on multiple forms on the same object it can only have one value for the object. Usually you report or filter based on the values of the fields, but the forms must be attached for the fields to be saved.