The Less than operator applies to attributes that can be ordered. It is used to select those contacts for which the relevant attribute has a value less than the one entered in the condition input field.
Setting the Number of children condition to Less than, with 3 as the input field value, selects all contacts with 0, 1 or 2 children, but does not select those with 3 or more children.
See Input field types for more about each input field.
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