Lore

Lore is specialized knowledge of a narrow topic, and it has many subcategories: Military Lore, Sailing Lore, Vampire Lore and so on. Each subcategory is its own skill, so ranks in Sailing Lore do nothing for Planar Lore. Every Lore uses Intellect.

Your background gives you one Lore subcategory. The GM decides what other subcategories to allow. They are always narrower than any of the other skills that can Recall Knowledge, and should never fully or mostly replace another skill's Recall Knowledge: Magic Lore wouldn't cover the breadth of Mysticism, Adventuring Lore wouldn't simply tell an adventurer everything they need, and Planar Lore wouldn't cover what's spread across other skills and narrower Lores such as Heaven Lore.

If several of your Lores (or a Lore and another skill) could apply to a check, use whichever has the better modifier, or the one you prefer. If there's any doubt whether a Lore applies to a topic or action, the GM decides. Even with no ranks in a Lore, you can use it to Recall Knowledge.

Related skills help. Ranks in Crafting, Medicine, Mysticism, Nature and Society each give 1 level of Advantage per rank on Lore checks in their fields (see those skills).

Common Lore Subcategories

You can learn any Lore your GM allows. These suit most campaigns, and backgrounds often grant one of them:

Academia Lore, Accounting Lore, Architecture Lore, Art Lore, Circus Lore, Engineering Lore, Farming Lore, Fishing Lore, Fortune-Telling Lore, Games Lore, Genealogy Lore, Gladiatorial Lore, Guild Lore, Heraldry Lore, Herbalism Lore, Hunting Lore, Labor Lore, Legal Lore, Library Lore, Mercantile Lore, Midwifery Lore, Milling Lore, Mining Lore, Sailing Lore, Scouting Lore, Scribing Lore, Stabling Lore, Tanning Lore, Theater Lore, Underworld Lore, Warfare Lore; Lore about a specific deity; Lore about a specific creature or narrow group of creatures (Demon Lore, Vampire Lore…); Lore of a specific plane (Astral Plane Lore, Heaven Lore…); Lore about a specific settlement; Lore about a specific terrain (Mountain Lore, River Lore…); Lore of a type of food or drink (Alcohol, Baking, Butchering, Cooking, Tea Lore…).

Recall Knowledge DCs

Ask the GM a question; the GM decides how obscure it is and rolls the DC:

Actions

Recall Knowledge (Concentrate)

You try to remember something about a topic. Ask the GM a question about the subject at hand; the GM decides how hard it is and rolls the DC: Common 10 + 1d6, Uncommon 10 + 2d6, Rare 10 + 3d6, Legendary 10 + 4d6.

Critical Success: You remember accurately, get extra information or context, and may ask one more question about the subject.

Success: You remember accurately, or pick up a useful clue about your situation.

Critical Failure: You remember something wrong, or get a false or misleading clue.