A frill is nei­ther an obvi­ous­ly non-func­tion­al aes­thet­ic touch (that would be a flour­ish) nor a clear­ly unnec­es­sary func­tion­al capa­bil­i­ty (that would be wast­ed func­tion­al­i­ty). Rather, it is a func­tion­al capa­bil­i­ty you want to sig­nal as being a part of your life, but not actu­al­ly use.