External map layers are represented as either:
These are available in the basemap card in the layers panel and are selected as any other basemap.
These are available in the Add layer selection. Unlike basemaps, overlays can be placed above or below any other overlay layers.
Overlay layers are represented by additional layer cards in the layer panel such as:
Along the top of the overlay card from left to right are:
A grab field to allow dragging and re-ordering layers with the mouse
The title of the external map layer
An arrow symbol to collapse and expand the overlay card
Along the bottom of the overlay card from left to right are:
A slider for modifying the layer transparency
A delete (trash can) icon to remove the layer from the current thematic map.
Here are some examples of external layers: