## 1 Introduction  
Customization plays a crucial part in the rendering of software as a service (SaaS) to various tenants [1, 2, 3], as the interface, business logic, and data are highly likely to vary for every tenant [4]. Thus, customization could engender performance threats which should be taken into account by the SaaS hosts [5]. Furthermore, all elements of a SaaS application (such as the design elements of the GUI, business practices, and databases) are impacted by tenant-oriented customization [6]. As such, a customizable SaaS application should consider the customization of all components of the SaaS application, including, for example, those with cross-layer relationships [5]. Additional consideration should be given to the likelihood that the software source code modifications necessary for customization will be rendered more and more complicated [1,7,8] by the necessity to separately preserve the customization code of every tenant [8]. Moreover, a tenant's requirement changes usually surface after the development of applications and services; hence, the run-time specific customization pertinent to a tenant should be provided during the same application instance [1,7,9,10], and it should not impact tenant isolation and application availability [7,10]. Usually, SaaS applications do not possess extensibility comparable to that of licensed software products [11]. Finer control towards customization would decrease application maintenance efforts [12,13], where certain maintenance jobs can be offloaded to the tenant-side (client-side). Nevertheless, extensive customization increases the regular maintenance requirements of any SaaS application, and this may increase scalability-related and cost-efficiency-related risks [1; 10]. Generally, the small investment made at the beginning and the monthly usage charges obtained from the tenants are insufficient to account for the total expenditure accrued by complex customization. Hence, SaaS vendors must remain vigilant to ensure they understand their customization expertise [12; 14] and gauge any customization's effect on the essential features of SaaS [1; 7; 15; 16]. These aspects must be accounted for before introducing customization requirements within a SaaS application. Previously-conducted research offers little information regarding the impact of software customization on SaaS quality [17]. Before investigating this impact, it is crucial to take note of the category of customization. This enables determination of the consequences and risks associated with a specific change [18], where, regardless of the type of customization, there is a chance that software product quality could be affected [19; 20]. For example, providing support for new requirements that are not yet covered by the SaaS application can have an effect on the software architecture [1], resulting in decreased quality of the application [21]. In the case that these new requirements are identical to requirements already provided for other tenants, it is important to update the predefined configuration options of the tenant every time a tenant's requirements change [1]. Such changes lead to ever-growing complexity, and under these circumstances, quality is likely to decline as the application grows [21; 22]. The specified obstacles to and shortfalls concerning SaaS customization have already provided the necessary motivation for researchers to formulate an initial software customization model for SaaS quality based on academic-related literature [23] and opinions of academic-related experts [24]. The model comprises (1) generic approaches used for software customization along with a set of common practices pertaining to every customization approach in the context of multi-tenant SaaS, and (2) essential quality attributes concerning SaaS customization. However, the empirical evidence on the association between software customization and SaaS quality has not been reported in the model.