"Standard SH3" uses whatever crew configuration files are already in SH3, usually the original SH3 files, but may be configs installed into SH3 by other mods.
"Realistic" uses an adjusted crew based on real U-boat crew composition.
"Data" appears for people that didn't follow the installation instructions.
A "Data" entry will not appear for anyone that fully uninstalled the previous version of SH3Cmdr prior to installing R2.4.