Break a card into child cards

Link cards under a parent, split its weight between them and see progress roll up.

In short

A parent card, such as an epic, can hold child cards. Each child owns a share of the parent, and the parent's progress is worked out from its children's. Children can sit on other boards of the same organization.

Before you start

Who: linking and unlinking children needs "Change cards" on the board. Choosing which card types are root types needs "Create and manage boards".
Plan: every plan.
First: you need access to both the parent and the child.

Steps

Open a card and its "Sub-items" tab (the child cards preview on Overview opens it).
Choose "Link existing" to find a card that already exists, or "Create child" to make a new one under it.
To change the split, choose "Edit shares", make them total 100 and press "Save shares". "Reset to even" hands it back.

On the same tab, press "Split into cards".
Paste the requirement or PRD. The card's description is there to start with.
Press "Make drafts". Each heading, top-level bullet or paragraph becomes a draft; what stands under a heading becomes its description.
Or press "Draft with AI": AI reads only the text you pasted and proposes cards with acceptance criteria and edge cases. It uses your own key or your AI allowance. If AI cannot help, the text is split as above and the reason is shown.
Rename, change or untick any draft, then press "Create". Up to 30 cards are made under the card at once, or none if one cannot be. A card you keep from an AI draft shows "Created with AI".

Result: the parent shows its children and a progress worked out from them.

Good to know

A card has one parent at most and can never be linked under its own child.
A child can sit on another board of the same organization; personal boards keep their cards to themselves.
Shares start as an even split that always totals 100 (three children read 34, 33 and 33). They stay even as children come and go, until you set them by hand.
A parent's progress is the share-weighted progress of its children. See /help/boards/progress-pace-and-health.
The rollup climbs up to 12 levels and stops at a card whose type is a root type, which you choose under Board settings, "Card types".
If the board estimates its cards, a parent also shows "Children add up to" a number of points next to its own estimate.
The Child cards section's "When will it be done?" line forecasts every card left below it, children and their children alike.
Unlinking leaves a child on its board with its own numbers and only stops counting toward the parent.
What happens next: the parent's progress and health are worked out again from its children.

If something goes wrong

"That card already has a parent. Unlink it first."
"That card is already above this one; linking it would make a loop."
"A child must be on the same board, or on another board of the same organization."
"The shares must add up to 100.": change the shares until they total 100.
"A card cannot be its own child."
"A hierarchy can be at most 12 levels deep.": link the card under a parent nearer the top.
"Every child needs a share." and "A share is a number from 0 to 100.": give each child a number in that range before you save.