By default, DITA2Go checks the outputclass attribute of each element for a format name, and also for possible border and shading format component names; see §6.2 Specifying options for naming formats:
[ElementOptions] ; UseOutputClassForFormat = Yes (default, use whenever present) or ; No (go on to mappings in [*FormatMaps] as the next choice) UseOutputClassForFormat = Yes ; OutputclassHasBorderShadeFormats = Yes (default, look for border and ; shading format specifications in outputclass attributes), or No. OutputclassHasBorderShadeFormats = Yes
To override border or shading properties for a particular instance of an element, you can specify different properties in the outputclass attribute for that instance.
When UseOutputClassForFormat=Yes and OutputclassHasBorderShadeFormats=Yes, DITA2Go checks outputclass attributes for multiple classes and separates them. If there are multiple classes, DITA2Go uses the first as the real outputclass (usually a block or inline format), the second as the border format, and the third as the shading format.
As an alternative, you can insert a BorderType or ShadeType PI marker in the element; see §38.1 Understanding DITA2Go PI markers.