A clustering index is defined on the fields which are of type Non-key and ordering.

Explanation:
A clustering index is a type of index that determines the physical order of the data in a database table. It is used to sort and store the data rows in the table based on the values of one or more fields. The clustering index is created on a specific field or group of fields in a table and is used to improve the performance of queries that involve range searches or sorting.

Types of Fields:
In the given question, the fields are categorized based on two criteria: key and ordering. Let's understand what these criteria mean:

1. Key Fields:
- A key field is a field that uniquely identifies each record in a table.
- It can be a primary key or a unique key.
- Key fields are used for indexing and enforcing data integrity constraints.
- In the context of the question, a key field is defined as a field that is part of the primary key or unique key of the table.

2. Ordering Fields:
- An ordering field is a field that determines the logical order of the data in a table.
- It can be used to sort the data in ascending or descending order.
- Ordering fields are used to improve query performance when sorting or searching for specific values.
- In the context of the question, an ordering field is any field that is used to sort or order the data in the table.

Clustering Index Criteria:
Based on the given criteria, a clustering index is defined on fields that are:
- Non-key: The fields are not part of the primary key or unique key of the table.
- Ordering: The fields are used to determine the logical order of the data in the table.

Reasoning:
A clustering index is created to physically sort the data based on the values of a specific field or group of fields. It allows the database system to store related data together on disk, which can improve query performance when accessing data sequentially or performing range searches.

If a clustering index is defined on a key field, it would result in a primary or unique clustered index, where the data is physically sorted based on the key values. However, in this question, the clustering index is defined on non-key fields, meaning the fields that are not part of the primary or unique key. This allows for the data to be sorted based on other criteria, such as an ordering field.

By defining the clustering index on non-key and ordering fields, the database system can efficiently retrieve data in the desired order, optimize range searches, and improve query performance for sorting operations.