Identify the business rules represented in the following ERD.

\begin{tabular}{|c|} \hline \multicolumn{2}{|c|}{ Invoices } \\ Invoice Number PK \\ Customer ID Number FK \\ Employee ID Number FK \\ \hline \end{tabular} \begin{tabular}{|l|l|} \hline \multicolumn{1}{|c|}{ PIANO } \\ \hline Serial Number \\ Mfg Completion Date \end{tabular}>011\begin{tabular}{l|} \multicolumn{1}{c|}{ MODEL } \\ \hline ID Number \\ Name \\ \hline \end{tabular}