Holacracy Community of Practice Archive, 2015-2019 Community Holacracy Web Site

In addition to what Tom said above, you could also define the optional nature explicitly. For example, "Doing X...if needed." 

Or, if the issue is that the objector doesn't want the role-filler to misinterpret the accountability as something to be done all the time, then you could use, "Doing X...upon request" or "Doing X...[when this certain condition occurs]."