## 7 Conclusions  
This article presents the outcomes achieved after performing an SMS. Our goal was to comprehensively review the current state of the art in the field and provide a synthesis of our findings with regard to RE activities in IoT software systems development. As a result, a total of 132,327 articles in the literature were obtained from essential scientific data sources. A total of 24 articles (primary studies) were finally studied in depth. The findings of this SMS clearly show that proposals have not been well-defined and created to satisfactorily conduct the development of IoT software systems through the application of RE phases. Moreover, the literature has not placed much relevance on this topic, and the techniques implemented have been inadequately applied. The significance of the comprehensive and accurate specification of requirements has been widely recognized because it aids in achieving an understanding with clients concerning software functionality and the central concerns in the development process. Figure 4: Primary studies published by year. Nevertheless, according to the results of this SMS, the phases of RE have not implemented in their entirety in IoT projects, primarily due to the particular features of the IoT, the multidisciplinary development groups involved, and the short periods required to release products into the market. Furthermore, IoT software systems can no longer be considered standard because of the diverse users accessing these devices or systems. Hence, offering proposals within the RE in the IoT field is necessary in order to improve the construction of this kind of solution, bearing in mind the large and dissimilar user population, as well as their requirements, desires, and goals considering the RE phases and NFRs. In this SMS, we also assessed trends over time. Our conclusions demonstrate a lack of proliferation of articles with new ideas in this field. We observed the continued application of traditional RE techniques, especially UML use-cases, metamodels, misuse cases, interviews, surveys, workshop sessions, questionnaires, goal-modeling oriented language, UML profiles, prototypes, UML activity diagrams, test cases, requirement specification documents, and interaction matrices. Nevertheless, we did not observe an increase in the number of articles building upon past work (implementations, integration, and extensions). Only two publications were extensions (improvements) of previous works. Overall, interest in RE for IoT systems has not increased. The definition of a complete SE methodology, considering the phases of RE, is a very complex issue and must be based on many authentic pieces of evidence. Furthermore, it must be accompanied by a good set of models and tools to represent and produce results, according to the fundamental principles of the paradigm. These conceptual and software artifacts will eventually lead to the development of the final product. To conclude, we found that 13 proposals considered some RE phases but only for a single type of IoT system, such as healthcare, disaster management, control, or building automation. Nevertheless, there were no generic proposals for use within the software development methodology for an IoT system that covered the generic RE phases of elicitation, analysis, specification, validation, and management. In this regard, as far as we know, only one proposal exists that covers all the RE phases described in Section 2; this was the primary study presented in [30], but it focused only on the development of a requirements specification document. Finally, we recommend some guidelines for future research obtained from our results. 1. The conceptualization of an approach to be used within a software development methodology for IoT systems, covering the generic RE phases of elicitation, analysis, specification, validation and management, is required;
2. There is a lack of support in the form of a methodological approach to guide the conceptual design of IoT software systems considering RE;
3.