Rat Meat is unique to "Extra Episode: The Struggle". In this extra story, it serves as a health item which, when enough are collected, provides the player with an extra life should they fail or die during a chapter.
Locations
It first appeares Chapter 1 "The Struggle".
Bibliography
Tada, Takuji, ed (2015). BIOHAZARD REVELATIONS 2 ULTIMANIA. Square Enix.