Update saves changes and usually returns you to the previous list. Save keeps the form open after saving. Insert creates a new record from the current values, while Insert and Stay keeps the new form open. The exact UI actions available depend on the form and your access.
Choose the action that matches the record outcome. Reusing an existing form as a template can accidentally alter the original if you choose Update instead of Insert. Conversely, inserting a copy when the request was to fix one field creates an unwanted duplicate. The simulator checks the resulting record rather than which button you pressed. A UI action may also change state or run server behavior, so read its label and confirm the intended record before saving.