An Opal outline can display a checkbox in every topic. You can use these checkboxes however you like; for example, you might use an Opal outline as a to-do list.
The checked state is automatically synchronized up and down the hierarchy, as follows:
If all of a topic’s daughters are checked, the topic itself is checked. The reverse is also true: if you check a topic, all of its daughters are checked (and this rule is applied automatically, so in fact the topic’s entire family is checked).
If all of a topic’s daughters are unchecked, the topic itself is unchecked. The reverse is also true: if you uncheck a topic, all of its daughters are unchecked (and this rule is applied automatically, so in fact the topic’s entire family is unchecked).
If some, but not all, of a topic’s daughters are checked, the topic’s checkbox is displayed in mixed state (a horizontal line).
To toggle the display of checkboxes in a document:
If checkboxes are showing and you hide them, their state is remembered, and will be restored the next time you show them. (But their state may also change while they are hidden if you rearrange topics, in keeping with the rules about automatically synchronizing checked state hierarchically.)
To check a checkbox:
Click in the checkbox.
Or, select a topic (or multiple topics) and choose Topic > Check Topic.
To uncheck a checkbox:
Click in the checkbox.
Or, select a topic (or multiple topics) and choose Topic > Uncheck Topic.