In the 'Types for Concrete Design' 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 means of 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.