A MODIFY request processes a transaction in three steps:

1. It reads a transaction for incoming data values. Transactions can come from external data sources, may be supplied by the user in screens or in response to prompts, or can be included as part of the request itself.
2. It selects a segment instance for changing or deleting, or confirms that a segment instance does not exist yet in the data source.
3. It changes or deletes the segment instance it selected, or adds a new segment instance.

This is shown graphically in the following diagram:

The request first reads a transaction (that is, a related collection of incoming data values). Describing Incoming Data describes the FIXFORM, FREEFORM, PROMPT, and CRTFORM statements that describe transactions read by the request.

After it reads a transaction, the request selects a segment instance in the data source to modify. It does this in either of two ways:

• It searches the data source for segment instances containing the same values as the transaction. This is done with a MATCH statement.
• It selects the next segment instance after the current position. This is done with a NEXT statement.

The MATCH and NEXT statements are discussed in Modifying Data: MATCH and NEXT.

The request then either adds, updates, or deletes data source values using the incoming values, or it rejects the transaction.