One label from each tree

By default a task can carry one label from each tree. The label at the top of the tree and everything under it count as one group.

That's what makes a label tree work like a field, with the labels under it as the answers. If you have a tree like this:

Billing
├── Billable
└── Non-billable

every task has exactly one billing status. When you assign Non-billable to a task that already carries Billable, Nydli replaces the old one. It asks first, so nothing changes silently.

Separate trees don't affect each other. A task can always carry one label from Billing and one from Client at the same time.

If a tree really does need several labels on the same task, like Design and Copy, you can allow that for that one tree. See Allowing several labels per task.