Does this answer your question?
Example 1: All of the selected cells have the Conditional Highlighting rule shown to the right of the table.
The checkmark in column E is a text character, inserted into the cell. Values in columns B, C and D are entered from the keyboard. Other cells in these four columns are empty.
Column F contains two cells whose data format has been set to 'checkbox'. Both are highlighted because a checkbox cell always contains a value, either 'true' or 'false', and is never 'blank'.

In the second example, there is no change in the data format of the cells, but there is a change in the rule for the selected cells in column F. The new rule is shown to the right of the table. It is set to highlight the cells that are checked (ie. contain 'true') and ignore any other value in these cells.

Regards,
Barry