This slot specifies the directionality of a reaction. This slot is used in slightly different ways in class Reactions and Enzymatic-Reactions. In class Enzymatic-Reactions, the slot specifies information about the direction of the reaction associated with the enzymatic-reaction, by the associated enzyme. That is, the directionality information refers only to the case in which the reaction is catalyzed by that enzyme, and may be influenced by the regulation of that enzyme.

The slot is particularly important to fill for reactions that are not part of a pathway, because for such reactions, the direction cannot be determined automatically, whereas for reactions within a pathway, the direction can be inferred from the pathway context. This slot aids the user and software in inferring the direction in which the reaction typically occurs in physiological settings, relative to the direction in which the reaction is stored in the database.