Like your 5 primary abilities, skills have a rank. Additionally, for every odd number rank you have in a skill, you can choose a speciality in that skill.
This is a first draft of the skill list, and specialities for each skill.
Academics – [dead languages], Geography, History, Mathematics, Natural Philosophy, Peerage
Arcane – Alchemy, Astrology, Legends, Hedge Magic, [religious traditions], [magical traditions]
Athletics – Acrobatics, Climb, Marathon, Sneaking, Sprint, Swim
Beasts – Dogs, Falconry, Horseback, Legends
Business-???
Craft – Carpentry, Farming, Smithing, Stoneworking, Weaving
Deception – Acting, Disguise, Forgery
Defense – Shield block, Dodge, [weapon parry]
Healing – Diagnosis, Disease, First Aid, Hedge magic, Herbalism, Lunacy, Surgery
Melee – [weapons], Unarmed, Wrestling
Mechanics – Clockworks, Jury-Rigging, Locks, Millworks, Siege, Triggers
Investigation – Library, Rumormonger, Search
Paranoia – Ambushes, Cons, <light sleeper>, Poisons, Spies, Stealth, Traps, Usury, Weather
Persuasion – Casual conversation, Lies, Propaganda, Sales, Seduction, Speeches, Torture
Ranged Attacks – [weapons, e.g. crossbow]
Tactics – Battle plans, Guerilla campaigns, Logistics, Morale, Scouting, Siege weapons, Skirmish
Travel – Captain, Caravaneer, [languages], Hiking, Navigator, Orienteering, Packing, Sailor
Wilderness – [terrains, e.g. forest, mountains], Foraging, Survival
You can substitute skills for abilities during challenges. You can bid skill superiority during challenge bidding. If one of your specialities is appropriate to the challenge, you can bid the speciality, at the same rank as the skill.