Searches issues with / without subtasks, additionally you can further refine your query by using a number expression.
A 'number expression' is made of an optional operator representing by its mathematical symbol and a (prositive/negative) number.
Let's see the allowed symbols...
Not equals to
Greater than or equals to
Less than or equals to
By default, this function only includes issues with at least one subtask even if you use the '<' or '<=' operator. You must explicity add 'issues NOT IN hasSubtasks()' clause to your query, if you also want to search for issues without subtasks.