In the "Reinforcement" folder of the navigator (see the Navigator Items Relevant to Concrete Design image), the "Concrete Durabilities" category contains one entry by default. Double-click that XC1 item to open the "Edit Concrete Durability" dialog box.
The environmental conditions are classified in EN 1992-1-1, Table 4.1. They have an effect on the minimum cover requirements of the reinforcement. The "XC1" exposure class represents a dry or permanently wet environment, which is supposed to be suited to the model. It would be possible to set a different class here or create different types of durability by using the
button.
The "Concrete Cover Options" tab provides several options to modify the structural class according to EN 1992-1-1, Table 4.3N and adjust the allowance for deviation. As the default settings are appropriate, click OK (or Cancel) to close the dialog box.