Event Definition Schema

When creating an event definition, the API uses the information contained in the schema object to create a data extension associated with the event definition. The schema defines the name, nullability, and default values of the fields to be included in the data extension.

The event definition schema must specify a subscriber key field. This can be a subscriber key or email address, depending on the account type. The field specified will be used as the contact key when invoking journeys.

SendableCustomObjectFieldThis field maps to the subscriber key (or subscriber id/email address). This field should hold, in most cases, the contact key. Journey Builder uses this field to populate the contact key when invoking journeys. Journeys require a contact key.
SendableSubscriberFieldValid options are SubscriberKey, SubscriberId or EmailAddress. Most API implementations use the SubscriberKey value.

[Supported Schema Field Types](https://help.salesforce.com/articleView?id=mc_es_data_extension_data_types.htm\)