Indeterminate state
Also called: Mixed state, Partially checked
The 'mixed' state of a parent checkbox, shown as a dash, when only some of its children are checked.
Definition
A 'Select all' checkbox should show mixed when some items are selected, checked when all are, and empty when none are. Screen readers announce it as 'mixed'.
In practice
You select 2 of 5 emails, so the 'Select all' box shows a dash instead of a tick.