To export a screen or a container, along with its respective states, right-click on the title in the screens/ containers menu and select 'Export...'.
You then choose the scale to export in, along with the suffix. You can choose to export all states, or the current state (i.e. the one you are currently editing).
To export a state, just open the dropdown menu next to the state title in the states navigation panel and select 'Export...'.
Export UI items
To export a UI item, multiple UI items or groups of UI items, right-click after selecting them and choose 'Export...'.