The RSSD device is the Adaptive Server database device where rs_init creates the Replication Server RSSD.
If you want rs_init to create the device, verify that the Windows System Administrator account can create the file you specify and that there is sufficient disk space for the file. Execute sp_configure devices in Adaptive Server to verify that the value for devices is high enough to allow you to add a new device. See the Adaptive Server Enterprise System Administration Guide for information about device requirements.