This functionality is only available for certain module packages. Info

Formula Editor

The formula editor allows for determining values or displaying conditions for text. The properties offered in this editor are context-sensitive, i.e., the displayed properties depend on the location from where you opened the editor. If you open the formula editor, e.g., from the label decoration editor, label properties are available; if you open it from a connectable object symbol, then connector-specific, connectable object-specific and terminal properties are available, etc.

The formula editor shows which value must be used last to be able to define the formula: