A radio link is a ledger in decibels: start with the transmit power, add every gain, subtract every loss, and compare what arrives with what the receiver can hear. Free-space path loss is computed as 20log10(dkm) + 20log10(fMHz) + 32.4 (ITU-R P.525).
Free space is the best case. This ledger assumes a clear path with Fresnel clearance. Terrain, buildings, and foliage add loss on top of it, which is why the module's field exercise models the real path over terrain before you trust a number here.