Roleplaying game mechanics are one of several tools used by participants to describe events in the shared reality of the game world. Potestas has highly abstract game mechanics that are used mostly for two purposes: defining character capabilities (for example, can I lift this, do I know this, can I do this) and arbitrating the outcome of conflicts (such as an argument or a fistfight). Using Potestas for either purpose tells you what happens in the game world, but does not meaningfully describe how it happens. Providing plausible and consistent descriptions of the game world events that are being modelled using Potestas is the shared responsibilty of the players, including the GM.
Skills tend to have a broad range of applications and utility, with few internal limitations. This was intentional, partly to facilitate the design of "renaissance man" characters whose broad competencies and expertise are thematically appropriate for an early modern fantasy game, and partly to reduce complexity. If a character's description indicates they have limitations on their competency or expertise, it is the player's responsibility to incorporate that limitation into Potestas. For example, don't include the Athletics skill training bonus on swim checks for the character who can't swim; add the Melee skill focus bonus only when the master swordsman is wielding a sword; don't have the storyteller use the Writing skill to create a new opera.
Abilities are, essentially, supernatural skills that can be powered in almost all cases by any of the three sources of power in the game world. Again, this was intentional, partly to reduce complexity and partly to support the widest range of thematically appropriate source material. For example, saints who curse their enemies, pagans priests who heal those who drink their purified blood, and warriors who turn into bears can all be modelled using Potestas. It is the player's responsibility to articulate thematically appropriate rituals, components, effects, and consequences for using their abilities. Players should also note carefully how some abilities are described. For example, you cannot set someone on fire using Change Fire, because that lets you change a fire into another type of fire: bigger, smaller, hotter, cooler. To set someone on fire, you use Change Air and Fire to transform the air around them into fire.
Skill challenges can be difficult to conceptualise, particularly for players used to game systems that provide a detailed tactical mini-game for resolving combat and other conflicts. Each participants in a skill challenge is expected to outline how their action could assist their side in winning the challenge, choose the most relevant skill that describes that action, and after determining their success or failure describe what their character did. Describing the outcome of skill challenges can present difficulties, particularly when players on the same side disagree on what winning means for the opponent. Such disagreements can occur even if a challenge is resolved solely by the use of one skill in one way. For example, the winning side used Melee to attack their opponent with weapons. Was the opponent killed by the final blow, or did they fall, too weak from their many wounds to continue fighting? Were they disarmed after a lengthy and bloodless duel? Did they realise they were about to lose, throw away their weapon, and surrender? It is the player's responsibility to negotiate with the other players for outcomes that are mutually agreed, if not always mutually satisfying.