There is a schema definition for endpoints which return primitive types
e.g : 
/server/admin/containers/{containerId}/cases/instances/{caseId}" :
 
_

{"schema" : \{ "type" : "string" }

,_
 
swagger code generator will generate this endpoint method and will have String return type.
 
But for few REST endpoints model(in UI)/schema(in swagger.json) is not defined:
 
"/server/queries/tasks/instances/process/{processInstanceId}" : {
      "get" : {
        "tags" : [ "Process queries" ],
        "summary" : "Returns task instances associated with a specified process instance.",
        "description" : "",
        "operationId" : "getTasksByStatusByProcessInstanceId",
        "produces" : [ "application/json", "application/xml" ],
        "parameters" : [ {
          "name" : "processInstanceId",
          "in" : "path",
          "description" : "process instance id to filter task instances",
          "required" : true,
          "type" : "integer",
          "format" : "int64"
        }, {
          "name" : "status",
          "in" : "query",
          "description" : "optional task status (Created, Ready, Reserved, InProgress, Suspended, Completed, Failed, Error, Exited, Obsolete)",
          "required" : false,
          "type" : "array",
          "items" : {
            "type" : "string",
            "enum" : [ "Created", "Ready", "Reserved", "InProgress", "Suspended", "Completed", "Failed", "Error", "Exited", "Obsolete" ]
          },
          "collectionFormat" : "multi"
        }, {
          "name" : "page",
          "in" : "query",
          "description" : "optional pagination - at which page to start, defaults to 0 (meaning first)",
          "required" : false,
          "type" : "integer",
          "default" : 0,
          "format" : "int32"
        }, {
          "name" : "pageSize",
          "in" : "query",
          "description" : "optional pagination - size of the result, defaults to 10",
          "required" : false,
          "type" : "integer",
          "default" : 10,
          "format" : "int32"
        }, {
          "name" : "sort",
          "in" : "query",
          "description" : "optional sort column, no default",
          "required" : false,
          "type" : "string"
        }, {
          "name" : "sortOrder",
          "in" : "query",
          "description" : "optional sort direction (asc, desc) - defaults to asc",
          "required" : false,
          "type" : "boolean",
          "default" : true
        } ],
        "responses" : {
          "200" : {
            "description" : "Successfull response",
            "examples" : {
              _"application/json" : "{\n  \"task-summary\": [\n    

{\n      \"task-id\": \"2\",\n      \"task-name\": \"Self Evaluation\",\n      \"task-subject\": \"\",\n      \"task-description\": \"Please perform a self-evalutation.\",\n      \"task-status\": \"Ready\",\n      \"task-priority\": \"0\",\n      \"task-is-skippable\": \"false\",\n      \"task-created-by\": \"John\",\n      \"task-created-on\": \"2016-04-05T15:09:14.206+02:00\",\n      \"task-activation-time\": \"2016-04-05T15:09:14.206+02:00\",\n      \"task-proc-inst-id\": \"2\",\n      \"task-proc-def-id\": \"evaluation\",\n      \"task-container-id\": \"myContainer\",\n      \"task-parent-id\": \"-1\"\n    }

,\n    {\n      \"task-id\": \"1\",\n      \"task-name\": \"Self Evaluation\",\n      \"task-subject\": \"\",\n      \"task-description\": \"Please perform a self-evalutation.\",\n      \"task-status\": \"InProgress\",\n      \"task-priority\": \"0\",\n      \"task-is-skippable\": \"false\",\n      \"task-actual-owner\": \"kiesu\",\n      \"task-created-by\": \"John\",\n      \"task-created-on\": \"2016-04-05T15:05:06.508+02:00\",\n      \"task-activation-time\": \"2016-04-05T15:05:06.508+02:00\",\n      \"task-proc-inst-id\": \"1\",\n      \"task-proc-def-id\": \"evaluation\",\n      \"task-container-id\": \"myContainer\",\n      \"task-parent-id\": \"-1\"\n    }\n  ]\n}"_
            }
          },
          "500" : {
            "description" : "Unexpected error"
          }
        }
      }
    },
 
For endpoints that do not have the schema, there will be a void return type in client code.