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Transaction data is data describing an event (the change as a result of a transaction) and is usually described with verbs. Transaction data always has a time dimension, a numerical value and refers to one or more objects (i.e. the reference data).
Typical transactions are:
Financial: orders, invoices, payments
Work: Plans, activity records
Logistics: Deliveries, storage records, travel records, etc.
Typical transaction processing systems (systems generating transactions) are SAP and Oracle Financials