## 3 Classification of the Reviewed Literature
### Application Domain and Their Incorporated Contexts
#### 4 Places, Travel and Tourism Domains  
The most important consideration in these categories is the proactive recommendations. Proactive recommenders are the systems that push a list of recommendations to users when their situations seem appropriate without the users' explicit requests. The systems are always in keeping track with the users to know the condition that will warrant recommendations. They determine when and how the recommendations should be displayed to the users. The authors of [83] have proposed a domain-dependent proactive model for mobile recommender systems and the authors of [84] evaluate the acceptance of proactive recommendations among users and demonstrate how the presence of recommended items can be properly notified, those results show, that compared to a notification-based solution, a widget-based solution is preferred by users.  
Recommender systems play significant roles in the domain of tourism. A lot of current researches focus on providing helpful information of a tourism site to visitors at the right time about the site itself and the various services nearby [25]. This is crucial because the world wide web (WWW) has transformed the tourism domain to the extent that most travelers now rely very little on travel agents [85]. In fact, recent studies indicate that travelers when onsite, search for free available information about the site rather than renting a visitor guide [25].  
Considering these six major groups of application fields that recognized the importance of CARS, it can be asserted that in the near future almost all application fields may adopt CARS.  
Once the domain of application is been decided, next is to identify the required contexts of both the users and the items. While the term \"context\" has been studied across various disciplines, each discipline tends to provide its idiosyncratic view. In the field of computer science, \"context\" is commonly defined as [86]:  
_\"Any information that can be used to characterize the situation of an entity. An entity is a person, place, or object that is considered relevant to the interaction between a user and an application, including the user and application themselves\"_.  
Another study [81], Suggest that \"context\" should be viewed as interactional and representational. Representational in the sense that the predefined sets of \"context\" are observable and do not significantly change over period. In contrast, an interactional view assumes that \"contexts\" are not necessarily observable and are subject to certain factors induced by user behavior. In the early work of [87; 88], \"context\" was classified into computing, user, and physical contexts. Based on a review conducted by [45], the contexts that are incorporated into recommendation processes in a digital library were classified into users', documents, and environmental contexts.  
In Table 2, we present the various application domains and their incorporated contexts. Based on the review, the incorporated contextual information in the RS can be viewed as either formalizing or identifying a place for the users and/or the items (_spatial context_) or characterizing a possessed attribute of the users and/or the items. The possessed attribute can either be for a period of time (_temporal context_) or permanent (_static context_).  
_Spatial contexts_ are those contexts that formalize the geographical situation or environment of users and/or items such as their location.  
_Static contexts_ are those contexts that do not change over time, and which affect the recommendation process such as gender, age, and identity etc.  
_Temporal Contexts_ contrary to static context, are those contexts that change over time and which are dynamic in nature like user's mood, user's current goal and social relations etc.