## 1 Introduction  
Requirements engineering (RE) is the foremost, human-centric, and crucial phase of the software development lifecycle, concerned with adequately eliciting, analyzing, validating, and managinguser requirements. In real-world scenarios, it is quite hard to correctly fulfil all the requirements, which ultimately leads to poor quality software or project failure [1].  
A controlled vocabulary is an organized collection of units of significance, i.e., terms that have determined and well-known meaning, without duplicates (synonyms) that can cause ambiguities or misunderstandings. Their purpose is to organize information (data pieces) in a structured manner, provide consistency, indicate semantic relations, making it easy to classify, query, and retrieve data [2; 3; 4].  
Although developers have extensive knowledge of software development methods, they often ignore the domain of the problem [5]. Lacking such type of knowledge may lead to problems such as missing important information or specifying ambiguous, contradictory, or incomplete requirements [6]. To reduce or mitigate such problems, it is essential to use adequate methods, techniques, and tools that can effectively deal with domain concepts and their relationships, helping the developers to understand their intricacies. Controlled vocabularies (CVs) may be used as one of these effective strategies to reduce problems related to domain concepts in RE, since they reduce ambiguities and complexities.  
There are some previous studies that have investigated the use of CV in RE [7; 8; 9; 10; 11]. However, none of these secondary studies captures all aspects, impacts and evidence that interest us. Our goal is to explicitly identify and categorize the different types of CV available, and the support they provide to activities in the RE phase of software development.  
For this work, we have chosen the systematic mapping study (SMS) as a research method [12; 13; 14], to identify and classify the available empirical evidence about the use of CVs in RE. The SMS research approach has been used successfully on different topics within the scope of RE [15; 16; 17; 18; 19; 20; 21].  
The research method will help to get a deeper knowledge about CVs and their impact in RE activities, helping to:  
* Detect research gaps (future research opportunities);
* Aid decision-making (practitioners) when selecting a CV or a tool;
* Better plan the RE phase, avoiding pitfalls.  
The specific contributions of this work are to:  
* C1: Identify and classify the CVs (RQ1) used in activities related to the RE phase (RQ2) of software development;
* C2: Identify the impact of CVs on the development process and the final product (RQ3);
* C3: Identify some demographic data such as active researchers, organizations and countries, and the most frequent publication venues (DQs);
* C4: Gather dispersed evidence providing a centralized source to facilitate research.  
The remainder of this paper is organized as follows: Section 2 presents the background and related work whilst Section 3 details the research method. Results and discussion are presented in Sections 4 and 5 deals with the validity threats. Finally, Section 6 presents the conclusions and proposals for future work.