Once "configure network management-data-interface" command is executed, the Management default route will be changed to route through the data interfaces. And If you are connected to the Management interface with SSH, your connection may drop. You must reconnect using the console port.
Also 1 interface will have only IP address assigned to it. so IP which you will be assigning to interface ethernet1/1 will be your management ip.
You can also learn more about Secure Firewall (formerly known as NGFW) through our live Ask the Experts (ATXs) session. Check out Cisco Network Security ATXs Resources [https://community.cisco.com/t5/security-knowledge-base/cisco-network-security-ask-the-experts-resources/ta-p/4416493] to view the latest schedule for upcoming sessions, as well as the useful references, e.g. online guides, FAQs.