Understanding EIRP
EIRP represents the total power that would need to be radiated by an isotropic antenna to produce the same signal strength in the direction of maximum antenna gain.
EIRP (dBm) = TX Power (dBm) - Cable Loss (dB) + Antenna Gain (dBi)
ERP (Effective Radiated Power) is similar but referenced to a half-wave dipole instead of an isotropic antenna. ERP = EIRP - 2.15 dBi. Many regulatory bodies specify maximum EIRP or ERP levels.