UI Elements are the smallest useful unit for describing page content, and do not contain other elements inside themselves.
Element usually exist by themselves, but they my also be grouped together with other elements of their kind. This forms a plural definition of an element.
The look and feel of the plural may change to show a relationship, and plural definitions can often share many of the same variations as singular definitions.
The plural of button is "buttons" a button wrapped inside "buttons" will automatically apply styling appropriate for a group. Many button variations can be used on plural definitions just like they would be used in singular.
For example, in this case each button will be large because we understand it is a part of the large button group, but the continue button has its own variations applied as well.