Is there an acknowledged standard for multiple selection with checkboxes?

1. How do I set a checkbox indeterminate?
2. What does a square in a checkbox mean?

How do I set a checkbox indeterminate?

A checkbox cannot be set to indeterminate state by an HTML attribute - it must be set by a JavaScript. This state can be used to force the user to check or uncheck the checkbox.

What does a square in a checkbox mean?

This third state is shown as a square or dash in the checkbox, and indicates that its state is neither checked nor unchecked. This is most often used when the checkbox is tied to a collection of items in mixed states.