I believe that sheet is documented. Under documentation -> pythonista modules -> ui module -> ui.view.present, it says
"View.present(style='default', animated=True, popover_location=None, hide_title_bar=False, title_bar_color=None, title_color=None, orientations=None)
Present a view on screen.
By default, a view is presented with a title bar that also contains a “close” button. If hide_title_bar is set to True, you can close the view with a 2-finger swipe-down gesture, or by calling the View.close() method.
Valid values for the style parameter are:
'full_screen': The presented view covers the entire screen.
'sheet': The view covers a roughly quadratic portion in the middle of the screen. This style is only available on iPad.
'popover': The view is presented in a popover. The popover_location parameter can be used to specify its position; by default, it is shown in the top-right corner. This style is only available on iPad.
'panel': The view is presented in a sliding panel, along with other accessories (e.g. the console).
'sidebar': The view is presented in a sidebar, next to the main editor view. The sidebar width will be determined by the current width of the view, but it cannot be wider than 200 points.
The optional popover_location parameter can be a 2-tuple (x, y) that specifies the point (in screen coordinates) from which the popover is shown (where the arrow points).
The orientations parameter is a sequence of strings that determines in which device orientations the view will auto-rotate. It is only used for fullscreen presentation. Valid values are 'portrait', 'portrait-upside-down', 'landscape', 'landscape-left', 'landscape-right'. By default, all orientations are supported."