https://tuleap.net/wiki/index.php?pagename=REST%20API%2FCardwall&group_id=101

In bold, the RW fields

{
  "id"            : <int>,               // The card id, not-null, read-only
  "label"         : <string>,            // The label, not-null(?), read-write
  "uri"           : <string>,            // The card RESt URI, not-null, read-only
  "project"       : <Reference>,         // not-null, read-only, Probably the same as the backlog item & the cardwall's milestone
  "artifact"      : <ArtifactReference>, // not-null, read-only, the artifact this bi represents

  // Card-specific
  "status"        : "Open"|"Closed"      // The status of the card
  "accent_color"  : <string>,            // Color as a string, format "#FFAAEE", exactly 6 hex digits preceded by a hash sign
  "column_id"     : <int>,               // nullable, if not null value is one of the IDs in cardwall's "columns" list
                                         // if null, the card is displayed on the left column, which has no id. It cannot be dragged elsewhere
                                         // if not null, it can be dragged to allowed columns
  "allowed_column_ids" : [<int>]         // Allowed columns, never null but possibly empty

  // The following list of values is a subset of the card's artifact values
  // Only fields to display on the card are transmitted
  "values" : [                           // not-null (but can be an empty list), read-write
        {
          "field" : {
                "id" : <int>,
                "label" : <string>
          },
          "value" : <string>,           // (cf tracker/field definition) only one of the following (value/bind_value_id/bind_value_ids/file_descriptions/links)
          "bind_value_id": <int>,
          "bind_value_ids": [
                <int>
          ],
          "file_descriptions": [
                {
                  "file_id": <int>,
                  "submitted_by": <int>,
                  "description": <string>,
                  "name": <string>,
                  "size": <int>,
                  "type": <string>
                }
          ],
          "links": [<ArtifactReference>]
          ]
        }
  ]
}