TEA here functions as an alkali. TEA is so weak as a chelating agent, it is not considered one as such and is often used in combination with a "real" chelating agent such as EDTA. A chelating agent can be considered a material which combines with specific metal ions and reduces their activity or prevents side reactions from taking place. A good place to learn about TEA and chelating agents is via Wikipedia.

The Sulfite is primarily a silver halide solvent and acts in a fashion similar to its role in D-76.

PE