This is the code page of the COBOL file (.cbl), not the data file. The code page must be compatible with the PowerCenter Client code page.
When you select this code page, the data file uses this code page by default. After you import the COBOL file, you can configure the code page of the source data when you adjust the source definition or when you run the workflow.
The Designer lists the five code pages you have most recently selected. Then it lists all remaining code pages in alphabetical order.