The braced keyword indicates that the element in question has a curly brace associated with it. Elements for which this is frequently necessary are <lg type="tercet"> and <castGroup>. The keyword does not take an argument, just a set of empty parentheses:
<lg type="tercet" rend="braced()">
At present we have no way of indicating which way the brace faces, or which side of the element it is on.