This paper presents a generalized stochastic geometry framework for analyzing the coverage and ergodic rate performance of integrated sensing and communication (ISAC) networks. The framework allows for the definition and calculation of coverage and ergodic rate metrics for both sensing and communication functions under resource constraints. The study investigates the fundamental limits of ISAC networks by considering the coupling effects of dual functions in coexistence networks. Theoretical results are derived for the coverage rate of unified ISAC performance, taking into account the interdependencies between sensing and communication functions. Additionally, analytical formulations are obtained for evaluating the ergodic sensing rate constrained by the maximum communication rate and the ergodic communication rate constrained by the maximum sensing rate. Numerical results validate the accuracy of the theoretical derivations and demonstrate that denser networks significantly enhance ISAC coverage. Specifically, increasing the base station density from 1 km⁻² to 10 km⁻² boosts the ISAC coverage rate from 1.4% to 39.8%. The results also show that increasing the constrained sensing rate significantly improves the communication rate, while the reverse is not as evident. The study provides insights into the optimal deployment of ISAC networks and highlights the importance of balancing the performance of sensing and communication functions in coexistence networks.This paper presents a generalized stochastic geometry framework for analyzing the coverage and ergodic rate performance of integrated sensing and communication (ISAC) networks. The framework allows for the definition and calculation of coverage and ergodic rate metrics for both sensing and communication functions under resource constraints. The study investigates the fundamental limits of ISAC networks by considering the coupling effects of dual functions in coexistence networks. Theoretical results are derived for the coverage rate of unified ISAC performance, taking into account the interdependencies between sensing and communication functions. Additionally, analytical formulations are obtained for evaluating the ergodic sensing rate constrained by the maximum communication rate and the ergodic communication rate constrained by the maximum sensing rate. Numerical results validate the accuracy of the theoretical derivations and demonstrate that denser networks significantly enhance ISAC coverage. Specifically, increasing the base station density from 1 km⁻² to 10 km⁻² boosts the ISAC coverage rate from 1.4% to 39.8%. The results also show that increasing the constrained sensing rate significantly improves the communication rate, while the reverse is not as evident. The study provides insights into the optimal deployment of ISAC networks and highlights the importance of balancing the performance of sensing and communication functions in coexistence networks.