Sum is a generic, universal rule system that has no associated genre or setting. The rules are simple and easy to use. All characters have narrative tags that are descriptive and drive character motivation and goals. Atypically, these narrative tags also have a value associated with them to indicate how "strong" the narrative tag is. Characters also have Abilities which collapse all traits into a single category (e.g., there are not Attributes and Skills in this system--there is only the single category of Abilities). Abilities have scores ranging from -5 to +5.
The core action resolution system involves establishing a target difficulty and subtracting that from the character's ability level. Then 2d6 are rolled, but one is treated as positive and one is treated as negative (that is, one helps and one hinders). These dice are summed to the previous value with negative results indicating failure and positive results or zero indicating success. The system supports varying degrees of success or failure. Opposed actions are similar except the opponent's ability and roll are contrasted to the actor's ability and roll instead of a target difficulty. The rules support a group action resolution method, as well, which is quite interesting.