## 2 Literature Review  
Urban interstitial areas, serving as crucial frontiers of urban functional agglomeration and diffusion processes, have increasingly emerged as zones of developmental significance amid ongoing decentralization and regional integration. These transitional spaces, known as \"gaps\" or \"interstices\", simultaneously benefit from positive externalities radiating from metropolitan cores and lower land costs. Initially, urban interstices were seen as passive or residual spaces at the peripheries of major urban centers, and they were often overlooked in mainstream urban theory [3; 6; 21]. Within contemporary globalization and polycentric urban restructuring, such distinctive spatial entities have attracted interdisciplinary scholarly attention with respect to geography, urban planning, and sociology [4; 5; 6; 8; 13; 18; 22; 23; 24; 25; 26]. A key theoretical framework underpinning the development of urban interstitials is the research of [PERSON] (1957), who is associated with classical regional economic theory; specifically, his concepts of \"diffusion\" and \"backwash\" effects are of particular relevance [27]. The research of the [PERSON] model describes how urban centers exert both positive externalities (diffusion effects) and negative forces (backwash effects) on the surrounding peripheral areas. Diffusion effects refer to the outward flow of benefits and development from urban cores, while backwash effects describe the inward pull of resources, labor, and capital toward these centers. The balance between these effects helps explain the uneven development of interstitial areas, where peripheral spaces experience either intensified development or underdevelopment depending on the relative strength of diffusion and backwash effects. This framework provides valuable insights into how urban interstices evolve (Figure 1), as they often experience both types of effects simultaneously--being shaped by the external influence of core cities while still maintaining distinct spatial and administrative characteristics [22; 28]. Following the research of [PERSON], more scholars have turned to the empirical realities of interstitial urban spaces, revealing that they are far from simply marginal, residual, or vacant [22; 29; 30; 31]. Instead, these spaces are increasingly recognized as loosely regulated yet highly plastic and relational areas that challenge conventional urban models. Their partial formalization, pending status, and embeddedness within infrastructure, governance, and ecological networks make them fertile ground for rethinking urbanization processes  
Figure 1: Mechanism of the urban function spillover effect. beyond traditional center-periphery dichotomies [22; 33; 34; 32; 35]. As [PERSON] and [PERSON] (2018) argue, interstices function across multiple spatial scales and reflect the unresolved tensions between planning and informality, and development and stasis [3]. These attributes position interstitial areas as laboratories for emergent urban forms and key sites for theorizing new urbanity under the conditions of decentralization, fragmented governance, and socioeconomic restructuring [4; 8; 14; 18; 36]. The shift from viewing these spaces as \"leftover\" to understanding them as productive and strategic urban frontiers has given rise to a growing body of interdisciplinary literature that reconceptualizes interstices as central to the contemporary urban condition [37; 38; 39]. Among this emerging body of work, several scholars have provided foundational conceptualizations that further illuminate the nature and significance of interstitial spaces. [PERSON], for instance, reconceptualized these areas beyond merely vacant or residual zones, emphasizing their role as dynamic environments characterized by distinct socioeconomic activities, ecological diversity, and functional evolution [18; 21]. Similarly, [PERSON] highlighted the multifunctional nature of urban interstitials, focusing on their role in providing ecological resilience and cultural preservation in the interfaces between cities [30].