Discretionary (8)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">discretionary</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for optional that I've seen before include "Available as a choice but not obligatory" , "Not mandatory" , "Not compulsory, discretionary" , "Voluntary - not required" , "Such as one can take or leave" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">discretionary</span>' is the definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for arbitrary that I've seen before include "Capricious" , "lacking principles" , "Based on whims or irrelevancies" , "Based on random choice" .)