# Using Opaque Fields and Message Sizes

When using RV ESB, you can configure and use Opaque field and message size settings. With the settings, Substation ES can calculate the size of data communication with the BES (CICS or IMS).

By default, the RV ESB uses the size defined in the message buffer definition to cater for outbound data. To configure Substation ES to use the message data size instead, you must define the message definition and the conversion rule for a recipe service in the appropriate manner for reply data to be sent efficiently over the network.

The conditions for this are listed as follows:

- Enter only one field in the output conversion rule.
- In the buffer definition, specify a buffer offset of zero.
- In the message definition:
  - In the output message, the Receiving field is the Opaque data type.
  - The Decimal indicator on the message has a value greater than zero.
- In the Conversion rule definition, the output conversion rule Field SZ is set to M.

The conversion then uses either the defined buffer size or the returned data size, based on the size field in the conversion rule, to create the outbound message.

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