<p>A detailed analytical description of the P-Q capability boundaries of inverter-based resources (IBRs) connected to a point of interconnection (POI) through a tie-line, consistent with the plant structure in IEEE Std. 2800-2022, is not available in the literature. In this article, equations are systematically derived to describe the P-Q capability boundaries of multi-inverter IBR plants at any reference point of applicability (RPA), fully incorporating the constraints on apparent power, voltage magnitudes, and active power. It is shown that voltage magnitude constraint gives rise to a pair of elliptical arcs, while active power constraint leads to a pair of parabolic arcs. The boundary curves are obtained considering the possible range of variation in voltage magnitudes at both the inverter terminals and the POI. At RPAs other than POI of IBRs with long tie-line, the parabolic arcs arising from the active power constraint closely approach—and effectively coincide with—their linear approximation. In addition, the paper proposes two applications of the P-Q capability analysis: dynamic reactive power management under plant-level voltage control operation and compliance assessment with minimum reactive power requirements as per IEEE Std. 2800-2022. The proposed analytical description can be used for applications that require the knowledge of P-Q capability of IBR plants.</p>