Abstract: Objective Against the background of new energy system development and coordinated energy security strategies, the role of deep underground space is expanding from conventional resource extraction to energy security support, energy-system regulation, and low-carbon geological storage. Underground gas storage, hydrogen storage, compressed air energy storage, and CO₂ geological storage all rely on the storage capacity, sealing performance, and long-term safety of underground space. However, existing studies have not clearly distinguished geological resource volume, theoretically usable space, and engineering-usable space. The relationship between geological carriers and application scenarios also remains insufficiently defined. In addition, key technical issues have not been systematically summarized. Methods This study draws on engineering research related to underground gas storage, salt-cavern compressed air energy storage, geological hydrogen storage, and CO₂ geological storage. Geological carrier properties, storage and sealing mechanisms, operating conditions, and risk-control requirements are systematically reviewed. Representative engineering cases are used to compare cavern-type, porous-type, and fractured-vuggy spaces in terms of boundary identification, engineering modification, injection-production operation, and long-term monitoring. The adaptability between different geological carriers and typical application scenarios is then analyzed. The main constraints affecting the transition from resource evaluation to engineering-usability evaluation are further identified. Results From an engineering perspective, deep underground space is classified into three types of geological carriers: cavern-type, porous-type, and fractured-vuggy. Their differences in storage boundaries, spatial identification, sealing mechanisms, and operational risks are clarified. The adaptability between geological carriers and typical application scenarios is established. It is further proposed that resource evaluation should shift from resource-scale estimation to engineering-usable space assessment. Geological resource volume, theoretically usable space, and engineering-usable space are defined as progressive evaluation levels. Conclusion The development and utilization of deep underground space depend on the compatibility among geological carriers, engineering scenarios, and long-term operating conditions. Future research should improve engineering-usability-oriented evaluation methods. Greater attention should be given to fine characterization of geological carriers, wellbore and caprock integrity evaluation, sealing degradation under thermal-pressure cycles, multi-source monitoring and early warning, and digital-twin-based operation. These efforts can support site evaluation, construction, operation, and safety management in deep underground space development.