12.3.3 Inlined and Displayed Flow Objects
A flow object that is to be formatted so as to produce a sequence of inline areas is said to be inlined.  A flow object that is to be formatted so as to produce a sequence of display areas is said to be displayed.  Instances of some flow object classes can only be inlined; instances of others can only be displayed; and instances of others can be either inlined or displayed.  In the last case, whether a flow object is to be inlined or displayed is controlled by the characteristics of the flow object or by whether the flow objects attached to its ports are themselves inlined or displayed. The class of a flow object determines for each port of that flow object whether the flow objects associated with that port shall be inlined, or whether they shall be displayed, or whether they may be either inlined or displayed.

NOTE 39

The included-container-area flow object described in section 12.6.16, Included-container-area Flow Object Class allows a flow object that can only be displayed to occur indirectly in a line without causing a break.  For example, one may wish to mix vertical Japanese in a line of English text without causing a break.