12.6.8 Line-field Flow Object Class
The line-field flow object class is inlined and has inline content. It produces a single inline area. The width of this area is equal to the value of the field-width: characteristic. If the content of a line-field area cannot fit in this width, then the area grows to accommodate the content and, if the line-field occurs in a paragraph, there shall be a break after the line-field.
It has a single principal port.
It has the following characteristics:

•  field-width: is a length-spec specifying the width of the area produced by the flow object. The initial value is 0pt.

•  field-align: is one of the symbols start, end, or center specifying the alignment of the contents of the field. The initial value is start.

•  writing-mode: is one of the symbols left-to-right, right-to-left, or top-to-bottom.  The direction determined by the writing-mode shall be perpendicular to the placement direction.  The initial value is left-to-right.

•  inhibit-line-breaks?: is a boolean specifying whether line breaks shall be inhibited before and after each area produced by this flow object. This applies only to line breaks introduced by the formatter to make lines fit in the available space. The initial value is #f.

•  break-before-priority: is an integer that affects whether a break is allowed before this flow object.  The break priority of a potential breakpoint is the maximum of the break-after-priority of the flow object immediately preceding the potential breakpoint and the break-before-priorities of the flow object immediately following the potential breakpoint, and any characters immediately following that character for which the drop-after-line-break?: characteristic is true.  A break shall be allowed at a potential breakpoint only if the break priority is even.  This characteristic is not inherited. The default value is 0.

•  break-after-priority: is an integer that affects whether a break is allowed after this flow object as described in the specification of the break-before-priority: characteristic. This characteristic is not inherited. The default value is 0.
A line-break shall be allowed immediately before and after a line-field used in a paragraph.