do {
        let data = Data(json.utf8)
        let response = try JSONDecoder().decode(Event.self, from: data)
        if let period = response.response?.output?.periods?.period {
            for aPeriod in period {
                let tDetail = aPeriod.details?.tDetail
                switch tDetail {
                case .tDetailElement(let singleElement):
                    print("It's a single element")
                    print("Std: \(singleElement.std) - Sta: \(singleElement.sta)")
                case .tDetailElementArray(let multipleElements):
                    print("It's a multiple element")
                    for aSingleElement in multipleElements {
                        print("Std: \(aSingleElement.std) - Sta: \(aSingleElement.sta)")
                    }
                default:
                    break
                }
            }
        }
    } catch {
        print("Error while decoding: \(error)")
    }


But in my opinion, with your current example, if would be simple to not have the `enum` since you have the same element (in an array of directly):

With model change (I didn't do the encode, in case you need it):

    struct Details: Codable {
        let tDetail: [TDetailElement]
        enum CodingKeys: String, CodingKey {
            case tDetail = "TDetail"
        }

        init(from decoder: Decoder) throws {
            let container = try decoder.container(keyedBy: CodingKeys.self)
            if let asSingleElement = try? container.decode(TDetailElement.self, forKey: .tDetail) {
                self.tDetail = [asSingleElement] //Transform it into an array of one element
            } else {
                let asArray = try container.decode([TDetailElement].self, forKey: .tDetail)
                self.tDetail = asArray
            }
        }
    }

   do {
        let data = Data(json.utf8)
        let response = try JSONDecoder().decode(Event.self, from: data)
        if let period = response.response?.output?.periods?.period {
            for aPeriod in period {
                if let tDetails = aPeriod.details?.tDetail {
                    for aTDetail in tDetails {
                        print("Std: \(aTDetail.std) - Sta: \(aTDetail.sta)")
                    }
                }
            }
        }
    } catch {
        print("Error while decoding: \(error)")
    }