Catalog API (CAPI) introduced a new request/response attribute “DetailsDocument” that accepts and returns a JSON object. As a developer, building on top of the APIs, you can send JSON object in CAPI StartChangeSet API request and get a JSON object in the response of DescribeEntity and DescribeChangeSet API. This capability will exist along side the current experience of sending and receiving a string object in the “Details” attribute of StartChangeSet, DescribeChangeSet and DescribeEntity APIs respectively.
Source:: Amazon AWS