<select> tag?
Select2 was designed to be a replacement for the standard <select> boxes that are displayed by the browser, so by default it supports all options and operations that are available in a standard select box, but with added flexibility. There is no special configuration required to make Select2 work with a <select> tag.
A standard <select> box can display nested options by wrapping them with in an <optgroup> tag.
Only a single level of nesting is allowed per the HTML specification. If you nest an <optgroup> within another <optgroup>, Select2 will not be able to detect the extra level of nesting and errors may be triggered as a result.
<optgroup> tags be made selectable?
No. This is a limitation of the HTML specification and is not a limitation that Select2 can overcome. You can emulate grouping by using an <option> instead of an <optgroup> and changing the style by using CSS, but this is not recommended as it is not fully accessible.
<option> and <optgroup> tags serialized into data objects?
Select2 will convert the <option> tag into a data object based on the following rules.
And <optgroup> tags will be converted into data objects using the following rules