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An API Group contains APIs - you can create as many API Groups as you need. From the APIs menu, you can see the currently defined groups, and create new API groups.
Note that in the top right corner, you can see how many APIs and versions are created within each API Group.
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If you press the Edit button in the toolbar, you get this screen:
Here, you can edit the OpenAPI definition directly on the left - any changes you make will immediately be visible on the right - and any eventual errors will be shown so you can correct them before saving.
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