When is the server also a client?

In passive FTP the client initiates both the control and data connections, so the server never originates a connection and never acts as a client.

In active FTP mode, the client opens a control connection to the server on port 21 and advertises a local port via the PORT command. The server then initiates the data connection FROM its port 20 TO the client's advertised port, making the server the originating party - the role of a client - for that data channel. This is the only standard protocol scenario where a server initiates a connection back to the client.

When receiving email via SMTP, the mail server passively accepts inbound connections from sending agents and does not initiate outbound connections for that transaction.

During a file upload the client establishes the connection to the server; the server remains the passive accepting party throughout.