In the original version of the Storyteller System used in the World of Darkness and Street Fighter: The Storytelling Game, difficulty is a number ranging from 3 to 10. When rolling a dice pool for an action, each die that meets or exceeds the action's difficulty is a success, and each that is lower than the difficulty is a failure. Additional successes rolled increase the degree to which that character succeeds in the action, although the exact details of this increased degree of success are often left to the Storyteller's discretion.
In the earliest versions of the Storyteller System, difficulty was not standardized, although guidelines suggested a broad and often vague range of appropriate difficulties. With the Revised versions of these rules (first introduced in Vampire: The Masquerade Revised Edition), difficulty is standardized at 6. However, in all versions of the system, other factors can increase or decrease the difficulty of a certain action. Regardless of difficulty, a 1 is always a failure, and a 10 is always a success.
Difficulty functions differently in the Revised Storyteller System used in the Trinity Universe, Exalted, and Scion. Here, the number needed to roll is the target number, which is always 7 (barring the use of certain specific Charms in Exalted). Difficulty is the modifier that increases the number of dice that need to meet or exceed the target number in order to score a success.
In the Trinity series, difficulty is a number ranging from +0 to +3 (or up to +4 in Aberrant) that increases the number of dice that need to meet the target number in order to get a success. For example, when attempting an action with a difficulty of +2, a character would need to have a 7 or better showing on at least three dice in order for the action to succeed. In Exalted and Scion, difficulty works similarly, in that it represents the total number of successes required. For example, an action with a difficulty of 3 requires 3 dice to show a 7 or better in order for the character to succeed.
In the Storytelling System, difficulty subtracts dice from a dice pool, effectively duplicating its function in the Trinity version of the Storyteller System.