The Retrieve command scans a document file for the given label and returns the corresponding expression.

If filename is not an absolute filename, the setting of currentdir determines what directory to search.

label is an internal label for the expression in the document and does not correspond to equation numbers.

In order to create a such a reference, use Insert>Reference in the Standard Worksheet Interface. This will allow you to look up an expression based on its equation number and automatically generate the internal label.