The Scenario > Earth Temperature and Receiver/Radar > Earth Temperature parameters are used in calculations in the same way: to calculate the System Noise Temperature for a Comm Receiver or a Radar Receiver. However, they are different in terms of how you can apply them to objects.
With the Scenario Earth Temperature, you can apply the value to all receivers and radars at the scenario level on the Scenario Properties > RF > Environment page, in the Environmental Data tab. You can use this value throughout the scenario for all receivers and radars that are configured to use a computed system noise temperature.
To explicitly change these values for an individual radar or complex receiver, use the Receiver > System Noise Temperature tab on the Definition properties page for that object. This will give you options to use a constant temperature (the default value is 290 K / 16.85 degC) or to compute the temperature. If you compute the temperature, the antenna noise temperature can be configured separately from other system temperature values. For the antenna noise, you can use the Earth Temperature as defined at the Scenario level by selecting Inherit from scenario, or you can explicitly define the value.
Why are there different methods to define this temperature? Suppose, for example, that you have a scenario in which most of your receivers are at low altitudes in a temperate region, but there is one receiver in the mountains and one in a far-away desert. You can use the scenario Earth Temperature for the majority of the receivers and then explicitly set the Earth Temperature of the mountain and desert receivers; or, you can just explicitly set the temperature of each receiver individually. These options will increase your fidelity when calculating System Noise Temperature for a Complex Receiver or a Radar.
For more information on all Scenario RF properties, go to the help page linked here:
RF Environment Properties
For more information on System Noise Temperature, go to the help page linked here:
System Noise Temperature
