Open the desired Elevation/Section View

In the left panel, select Add Reference Level.

A reference level will appear with the cursor. Select the line in the elevation for which you wish to add a reference level.

Automatically a reference line will be set for that, and another reference level will be added to your cursor to add your second level. To exit the adding mode, press the Esc key.

To edit the reference level, click on the targeted level, and in the left panel that opens, you can specify the name (Under Name section), and under the Elevation field, you can determine starting level from the From dropdown menu and type in the elevation above that starting level into the Elevation field.

To delete the reference level, click on the targeted level, and press Delete key or select Delete Reference Level in the left panel that opens.