Infusion

Infusions are one of the ways that players can upgrade themselves in BioShock Infinite. When Booker comes across an Infusion he must choose between increasing his shield capacity, health capacity, or salt capacity. Once he picks one he cannot change it back.

Sources

http://www.playstationlifestyle.net/2013/01/14/bioshock-writer-on-class-specializing-and-infinites-traditional-rpg-elements/