What is an “Unrestricted Submarine Warfare”?

You technically should be able to tell based on context... it's exactly what it sounds like.

Explanation:

Unrestricted submarine warfare is a type of naval warfare in which submarines sink vessels such as freighters and tankers without warning, as opposed to attacks per prize rules (also known as "cruiser rules").

answer; the confederacy could afford to move a large number of prisoners hundreds of miles but could not afford to fee and house the prisoners;