An active Steuard license gives you two things for its term: automatic updates and support. When a new version ships, it appears on your Updates screen like any other plugin, and you can reach support while the license is active.
Steuard is a single product — every feature is included. There is no separate "Pro" version to buy. The paid tiers (Personal, Business, Agency) differ by how many sites you can activate and the level of support, not by which features you get.
The plugin's code is GPL and stays yours to use regardless (see What software license does the plugin use?); the license is specifically the updates-and-support service. If a license lapses, the plugin keeps working, but you stop receiving new versions and support until you renew.
To enter or move a key, see Activating Your License.
