How can i make an Equation that relates a dimension of a component of an assembly with the dimension of the other component

How can i make an Equation that relates a dimension of a component of an assembly with the dimension of the other component, i saw thsi on a training file, i am currently studying "Solidworks Advanced Topics", the equation goes like this ="D1@Extrude1@Base1-1.Part"

SolidworksAssemblies