From the documentation:
getCurrentOrientationAnchoredAdaptiveBannerAdSize
(anchored adaptive banners)
When to use adaptive banners
Adaptive banners are designed to be a drop-in replacement for the
industry standard 320x50 banner size, as well as the smart banner
format they supersede.
These banner sizes are commonly used as anchored banners, which are
usually locked to the top or bottom of the screen. For such anchored
banners, the aspect ratio when using adaptive banners will be similar
to that of a standard 320x50 ad, as can be seen in these screenshots
getCurrentOrientationInlineAdaptiveBannerAdSize
(inline adaptive banners)
When to use inline adaptive banners
Inline adaptive banners are larger, taller banners compared to
anchored adaptive banners. They are of variable height, and can be as
tall as the device screen.
They are intended to be placed in scrolling content