Javadoc
If your cache has more zoom levels than you want to provide access to
with this layer, supply a zoomOffset. This zoom offset is added to the
current map zoom level to determine the level for a requested tile. For
example, if you supply a zoomOffset of 3, when the map is at the zoom 0,
tiles will be requested from level 3 of your cache. Default is 0 (assumes
cache level and map zoom are equivalent). Using zoomOffset is an
alternative to setting serverResolutions if you only want to expose a
subset of the server resolutions.