Reification
An application lists invoices that are more than 30 days overdue. The business changes that threshold to 60 days. For the same evaluation date, an invoice 45 days overdue disappears from the result even though its data is unchanged. Explaining the difference requires the query's earlier definition as well as the invoice data.
If the query exists only in application code, finding that definition requires connecting the result to the code that ran. A deployment record can help identify the revision, but the application still has to connect those records. Keeping the query in the database gives it a place beside the invoices. Its definition becomes something the application can examine directly.
Reification gives a concept or action an identity and represents it as data. A stored query has a definition that can be retrieved and changed, and its changes have transaction history. The overdue threshold therefore belongs to a definition with revisions that the application can inspect. Earlier data and the definition used to interpret it can be examined within the same database.
Definitions
The same principle applies to schemas and mutations. A schema describes the requirements an operation places on data, while a mutation defines how selected data should change. Their definitions are also part of the database model. The application can query that model to examine the behavior it depends on, using the machinery it already uses for its records.
Keeping definitions as data also lets behavior operate near the records it uses. A stored mutation can select and change records without sending every intermediate result through application code. Its definition remains available for inspection and revision even though the database executes it. This combines proximity to the data with a representation the application can manage.
Transactions
A change is itself something the application may need to describe. When the overdue threshold changes, the transaction that records the new definition has its own identity. The application can attach context to that transaction, such as the reason for changing the threshold. A query can connect a changed invoice or query definition to the transaction and its recorded context.
Definitions that control behavior need the same care as application code. A query change can alter the meaning of a report, and a mutation change can affect many records. Keeping their revisions makes those changes available for review without establishing that they are correct. Validation and control over who can change a definition remain necessary.