Hello,
as far as i know you can then choose between a modded US "Run silent run deep campaign", and a german U-boat "Monsoon campaign".
It's both installed on your harddrive that way.
If you are sure you never want to ride in a US boat, you can spare this RSRD mod, and the patch. Or the other way round. Or whatever.
Hope i'm right
Greetings,
Catfish