Manual · Content and homebrew

Structured content

Which fields become mechanics, which sentences remain text, and what the engine deliberately leaves on the card.

Fields, not sentences

Every class, species, feat, item, spell and condition is a compendium entry: an id, a ruleset and a list of effects. One function derives the sheet from whatever entries a character holds. There is no separate homebrew engine.
A feature's entries is card text, read as written. "You have Advantage on saves against being frightened" with nothing beside it prints the sentence and does nothing.
Structured fields derive on their own: a species' speed, darkvision and resist; a background's skillProficiencies; a spell's damage and its scaling; an item's AC bonus; a class's slot table, when it sits under rowsSpellProgression.
Everything past that is a _sheetling block: a tracker, a toggle, a bonus, a rider, a button, a pick.
Two rules pick the effect kind. modifies when the number already exists with a base (AC, a save, speed, attunement slots). A granting kind when the thing does not exist until something grants it (grants_movement, grants_resource, grants_action).

File	Recognised by	Lands
Sheetling export	JSON with `"app": "sheetling"`	one character on your roster, portrait inlined; `sourceRefs` names the library files it needs
5eTools homebrew	JSON with `_meta.sources`	your Library
Fight Club 5 compendium or backup	`.xml` or `.backup`	the compendium to your Library, each embedded character to your roster
Game Master 5 campaign	the same	creatures to your Library, the campaign to a party you run
Game Master 5 PC or NPC file	the same	one creature to seat, never a playable character

One Import a file… button reads all of them. The preview lists each part with a checkbox and a destination; a part with nowhere to go is greyed with the reason.
The ruleset boxes: Offer to 2014 characters and Offer to 2024 characters. The one the file declares is pre-ticked. Ticking the other is allowed, and the dialog warns when the file argues against it: fixed ability increases on a species, an Origin feat on a background, a subclass feature at level 1.
Two files defining the same name resolve to whichever the library registered first, and that order is not yours. A shipped entry always wins over homebrew of the same name. Name your own things something new.

"N of M spells derive something" counts produced entries with at least one mechanic. Not imported is a raw entry that produced nothing, with its reason. Imported with a detail dropped is an entry that exists with one field the importer could not carry.
A produced entry is not a reachable one. A subclass no class's subclass level names is invisible on every sheet. Build a character with the file; the preview alone cannot tell you this.
With Node, from a checkout of the repository, in packages/rules:

npm run probe:5etools -- <file.json>
npm run probe:fc5 -- <file or folder> [--ruleset=2024] [--derive]

The 5eTools probe builds a character of the parent class at levels 3, 10 and 20 for every subclass in the file and says which trackers and toggles reached the sheet. It exits non-zero on any drop.
A spell pick's {@filter} can hold level, class, a Ritual restriction and a spell-attack restriction. school, subclass, source, search or a negated value drop the pick with a row. A filter the importer cannot hold never widens the pick.

What you import lives in your account only. A manager shares a file with a party from its Homebrew tab; members get a synced copy under the same id, and only that party sees it. Stash keeps a shared file's items out of players' compendiums until handed over at the item counter.
"Why can't I see this option" is almost always a library question: whose account holds the file, and is it still shared.

Never derived: what happens to the creature you hit, turn structure, cover, a Reaction that reduces damage after a hit, suppressing an attacker's roll, a spell's range or a change to it, a random chance to skip a cost, and any duration. The round counter counts what the DM typed.
Looks missing, ships already: a cap above 20 (modifies takes a max), a rider's die count growing by level (extraDice.count scales), charges that recharge at dawn (a tracker with a rolled reset), a condition granting Advantage (most already do).
For the stated half of "attacks against you", incoming_attack_fact prints the sentence on the Combat section, gated on a toggle if you like. Otherwise the card's text, or a text override, is the whole implementation.
Three misreadings that give too much. "Regain up to 4" is a choice from zero to four, not a grant of four. "If you have no uses left" fires at zero; it does not refill early. "Unlimited" is no max at all, not 99.