Double Brackets [[ ]] vs Double Braces {{ }} in Polymer

What's a succinct way to explain the difference between double brackets ([[...]]) and double braces ({{...}}) in Polymer 1.0?

For instance, in the documentation for the <iron-list> element the sample HTML shows: <template is="dom-bind"> <iron-ajax url="data.json" last-response="{{data}}" auto></iron-ajax> <iron-list items="[[data]]" as="item"> <template> <div> Name: <span>[[item.name]]</span> </div> </template> </iron-list> </template>

Why is data bounded by double braces in one spot (last-response="{{data}}") but bounded by double brackets (items="[[data]]") in another spot?