When running network cable, it is important to evaluate the area you are running he cable in. For example, running cable above a drop ceiling, like in the one pictured below, may require we use special cable. If the area above the drop ceiling is used for active circulation of air it is known as a plenum space.
For all plenum spaces we are required, by fire code, to use plenum rated cables. Plenum rated cable uses a low smoke plastic sheath. That way if the cable is melting or burning it will not circulate smoke into the area where people may be present trying to escape the building because of the fire. See the box of cable pictured below and note it says "Plenum" on it.
Now if air is not circulating in the area above the drop ceiling you would still want to use plenum rated cable in case the air flow configuration changes in the future or if it becomes a plenum space because the air ducting leaks or come apart above the drop ceiling.