Custom attributes are legacy functionality only available via custom code written by a STEP developer. When custom attributes exist, the Custom Attributes data source is used to export the data.
The user interface for the mapping step of the Export Manager tool and the outbound integration endpoint tool are not exactly the same. For Export Manager, mapping is described in the Export Manager - Map Data topic here. For OIEPs, mapping is handled in the Output Templates section under the Format parameter as described in the OIEP - Event-Based - Output Templates Section topic (here) or the OIEP - Select Objects - Output Templates Section topic (here).
For more information about the additional wizard steps, refer to the Creating a Data Export topic (here) or the Creating an Outbound Integration Endpoint topic (here).
After mapping, most output options can be altered using transformations. For more information, refer to the Outbound Map Data - Transform topic here.