# Classification of Systems and Maintenance Models
## 2 Literature Review  
Most studies on the evaluation of maintenance indicators deal with performance metrics that fall into two groups: those that show how maintenance affects a system's overall performance, and those that address a system's operational reliability and availability. The long-run average profit per unit time of using the system is representative of the first group of indicators. Well-known representatives of the second group are achieved and inherent availability. The literature on the classification of maintenance effectiveness or performance indicators is abundant. Reviews of the classification of maintenance performance indicators can be found in [14; 15; 16]. In summary, the study [14] concludes that successful maintenance performance indicators should concentrate on assessing total maintenance effectiveness [17]. Seven categories of maintenance performance indicators that measure the overall maintenance effectiveness were included in this study [18]. Cost-, equipment-, and maintenance-task-related indicators are the most crucial. The study [15] examined the development of maintenance performance measures concerning the crucial maintenance organizational function and its resources, activities, and practices. According to a study [16], lagging measures, such as maintenance cost and safety performance, dominate the measurement of maintenance performance. Leading indicators such as maintenance work processes are used less frequently. The findings revealed no connection between the indicators employed and the maintenance goals pursued. The study in [19] used the original ELECTRE I, a multi-criteria decision-making method, to present a novel method for selecting maintenance key performance indicators. Following evaluations based on the key criteria, the suggested methodology generates a ranking of potential options. The study in [20] considered three levels of maintenance control: strategic, tactical, and operational. A set of maintenance performance indicators can be considered at each maintenance level. This study concentrates on maintenance performance indicators that support decisions at the operational level. The balanced scorecard is a different and comprehensive approach to measuring maintenance performance, as suggested in this study [21]. It is based on the idea that no single measure can accurately reflect the overall maintenance performance and that multiple measures must be used in conjunction. Let us consider the published indicators of maintenance effectiveness used in the operational stage. The study in [22] used instantaneous and steady-state availability as maintenance effectiveness indicators. The best inspection and imperfect maintenance policy that reduces the average long-term cost rate is then obtained using availability models. The study in [23] considered an analytical model for the steady state and instantaneous availability of the system. The authors determined the best method to change the inspection interval to maximize the steady-state availability of the system. A study [24] considered steady-state availability as a critical metric for telecommunication services in which network functions are handled using software. To determine the lower confidence limit of the availability of complex control systems, the study [25] presented a new availability assessment approach based on the goal-oriented method. The study [26] looked at a continuously tested digital electronic system that could have one of three failures: revealed, unrevealed, or intermittent. A new maintenance model for determining inherent availability was proposed. A mathematical model to describe mission availability for a system with bounded cumulative downtime was proposed in [27]. The suggested approach simultaneously considers cumulative uptime and downtime as restrictions. The study [28] considered a mathematical model to compute the operational reliability of an avionic line-replaceable unit (LRU), the probability of LRU recovery, and the maintenance cost.