The Choose Target Child Menu
Among other features, the presentation pane provides controls that allow you to edit the css rules of the various elements that make up your form. By choosing a parent element (in the Choose Parent menu) and then targeting a specific child type in the Choose Target Child menu, you tell the editor exactly which form elements you want the css rules you're editing to apply.
The Choose Target Child Menu contains a list of all the potential child types of the parent element
No matter the parent chosen, the first item in this list always represents the parent element. In the above case, selecting "This Form" from the Choose Target Child menu will allow you to edit css rules that will apply directly to the form itself.
Selecting "Child Fieldset" will let you edit css rules that will apply to all fieldsets that are children of the form, "Child Input" all html <input> elements that are children of the form, etc.
Error Division
Choosing "Error Division" will let you edit the css rules that apply to the error message that's generated when user input to your form violates restrictions you've placed on it in the behavior pane
Editing Container Rules
When you select a Form Element or HTML Element from the Choose Parent menu (as opposed to a Form Group like a fieldset), the Choose Target Child Menu might look like this
As explained in the presentation pane documentation, each Form Element and HTML Element is actually contained in (is a child of) an html paragraph element. This paragraph element is called a 'Container' and choosing "This Container" from the above menu allows you to edit the css rules that will apply to this paragraph element. In all other ways, the container is handled transparently by the editor, it's only identified here so you can apply css rules to it if you desire.
Choosing "Child Label" will apply styles to the label of this element and "Child Input" (or "Child TextArea", "Child Select Menu", etc.) will apply styles directly to the element.
Input vs. Text Box
To make creating Form Elements in the content pane easier, the html <input> element is divided into several Form Element types. Because the names used for these types (Text Box, Check Box, Radio Button, and Button) are only relevant in formArchitect and do not represent valid targets for css rules, they are not used in the Choose Target Child menu.
Instead, they are generically referred to as 'Input' in this menu and you'll see "Child Input" as opposed to "Child Text Box", "Child Button", etc. here.