Non-Functional Requirements (NFRs) are recognized as a critical factor to the success of software projects because they address the essential issue of software quality. NFRs tend to interfere, conflict, and contradict with one another and this conflict is widely acknowledged as one of the key characteristics of NFRs. Several models of NFRs conflicts have been proposed and the interacting nature of NFRs has been characterized as either positive or negative inter-relationships among NFRs. Positive relationship represents a pair of NFRs that are supporting each other while negative relationship represents those NFRs that are conflicting with one another. Furthermore, as NFRs are also relative, the interpretation of NFRs may vary depending on many factors such as the context of the system being developed and the extent of stakeholders' involvement. The multiple interpretations of NFRs may lead to positive or negative inter-relationships that are not always obvious. These relationships may change depending on the meaning of NFRs in the system being developed. Hence, the existing potential conflicts models remain in disagreement with one other. This paper presents the result of an extensive and systematic investigation of the extant literature over the notion of NFRs and the conflicts among them. Rigorous synthesis of the carefully reviewed literature has resulted in the construction of a catalogue of NFRs conflicts with respect to NFRs relative characteristic. The relativity of conflicts is characterized by three categories: absolute conflict; relative conflict; and never conflict. This comprehensive catalogue could assist software developers with identifying the NFRs conflicts, performing conflicts analysis, and suggesting potential strategies to resolve these conflicts.