One value per tree

By default, a task can carry one label from each tree — the tree's top-level label and everything under it count as one group.

This is what lets a label tree act like a field with values. With a tree like:

Billing
├── Billable
└── Non-billable

every task has exactly one billing status, never both. When you assign Non-billable to a task that already carries Billable, Nydli replaces the old value — it will ask first, so nothing changes silently.

Separate trees are independent: a task can always carry one label from Billing and one from Client at the same time.

If a tree genuinely needs several values on the same task — tags like Design, Copy, Research — you can allow that per tree. See Allowing multiple values.