Commercial certificates enable the electronic signing of documents. However, it is not possible to use them for communication with state authorities. Communication among organizations is the matter of their mutual agreement. Commercial certificates are used in the area of encrypted communication where the qualified certificate cannot be used (due to its inherent limitations).
If you wish to send an encrypted e-mail to someone, you can encrypt the content of your message using the public key of your counterparty's commercial certificate.
Furthermore, commercial certificates are used for network applications for establishing secure connections.