Load save files
File type is still detected automatically from contents, not which side you drop into — the two columns are just to keep things organized.
GameCube
Drop both the system file (unencrypted; supplies the encryption seed) and the character file together, or drop just the character file and enter its round-1 seed in the seed field on the Dreamcast side (shared between both platforms). GameCube saves still need the seed; only Dreamcast saves can supply their own.
Don't have a save file yet? How to get one off a real GameCube.
Playing Episode III: C.A.R.D. Revolution? Its saves use a different format — open the Episode III card editor.
Dreamcast
Drop the character .vms file, or a whole VMU memory card image (.bin/.vmd/.VMU). The seed is recovered from the save itself — you don't need the serial number.
Don't have a save file yet? How to get one off a real Dreamcast.