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Using the new Liferay Data Engine like described here
https://portal.liferay.dev/docs/7-2/frameworks/-/knowledge_base/f/data-engine
I have two questions and one issue:
1. Creating a DataRecordCollection in a DataDefinition seems to create 2 different collections.  Inspected with a GraphQL Query:

{
  "data": {
    "dataRecordCollections": {
      "items": [
        {
          "id": 37043,
          "dataDefinitionId": 37037,
          "dataRecordCollectionKey": "37042"
        },
        {
          "id": 37039,
          "dataDefinitionId": 37037,
          "dataRecordCollectionKey": "FIELDDEFINITION"
     }  
 }
}

The dataRecordCollectionKey "FieldDefinition" is the DataDefinitionKey of the corresponding DataDefinition.  BTW the request that creates the collection(s) returns the id of the first one. (In this example 37043)
Why are there two and which one must I use to finally store my records in?
2. Does anybody know an example that uses the property 

nestedDataDefinitionFields

?
What is it good for? Is it possible to store some kind of relation to other DataDefinitions?
3. After playing around with the Data Engine for a while ... (creation and deletion of dfinitions and collections) It seems that I can't delete DataDefinitions and DataRecordCollections anymore. The server responds with 500 (internal error).
Using Postman like this:

DELETE  http://localhost:8080/o/data-engine/v1.0/data-definitions/37028

reveals

com.liferay.dynamic.data.mapping.exception.NoSuchStorageLinkException: No DDMStorageLink exists with the key {classPK=37132}

in the response. Nothing in the Logs and restarting the server does not help.
Any advice?