When evaluating RF performance, it is typical for product data sheets to list several ranges for different environments. The reason for listing different ranges is that although radio waves can pass through objects, allowing for non line-of-sight communication, obstacles will absorb a portion of the signal greatly reducing the effective communication range.
As an example, the specifications for the Digi XBee-PRO module note an indoor/urban range up to 100 meters, while the range in outdoor line-of-sight conditions is listed up to 1.6 km. The difference in range is due to the environmental conditions and the amount of signal that is absorbed by the different obstacles. In order for radio waves to travel the greatest distances, the waves need to travel through as few obstacles as possible. Dense obstacles such as concrete, brick or other heavy construction will inhibit signals more than light obstacles such as a few trees or a simple post.
When trying to increase the range, several factors including antenna height, frequency, antenna type and power output all should be considered. Antenna height allows the signal to travel above obstacles and also reduce any interference from the ground. Lower frequencies travel farther than higher frequencies, and directional antennas can focus energy, thereby increasing range in fixed installations.