Multiple values at both levels

Setting	Where to edit it	Example
Outer cardinality	Conference → Manage fields → Sessions	Unlimited sessions
Sub-field cap	Compound fields → Conference session → Manage fields → Topics	3 topics per session

Build and try it

1. Create Conference session with unlimited Sessions and Topics capped at 3.
2. Add Opening keynote, with topics Community and Accessibility.
3. Use Sessions' Add another item button. Add Practical Drupal, with topics Field API and Content modeling.
4. Save and reopen. Check that each title keeps its own topics.

A cap limits the count; requiredness is separate. Increasing a cap adds nullable columns. Decreasing it can delete current and historical values and requires structural-change consent. A sub-field capped above 1 is an API array, even with one value.