CICS applications on the mainframe run with an EBCDIC character set. PCs use an ANSI character set, while on UNIX the character set in use is ASCII. When CICS applications are migrated from the mainframe, the majority of them are converted to use ANSI or ASCII, and they run under Enterprise Server using ANSI or ASCII. However, there are some cases where an application needs to continue to use EBCDIC on the server, while the client program uses ANSI or ASCII. In these cases, you need to set up data conversion macros to ensure the data is converted.
For details see the section Data Conversion in the chapter Legacy Extension with MSS in your MSS Configuration and Administration Guide.