Clarify useTileLayerAsFallback #222

Author: m-mohr

README.md

@@ -122,7 +122,7 @@ by using stacProxyUrl you can proxy the original STAC server with one that enabl
 
 ### tileSourceTemplate
 
-The `tileSourceTemplate` variable controls the tile layer that is used to render COGs.
+The `tileSourceTemplate` variable controls the tile layer that is used to render imagera such as (cloud-optimized) GeoTiffs.
 
 The format of this value is a tile layer template with an optional `{url}` that will be replaced with the COG asset href. For example,
 using a local version of [titiler](https://github.com/developmentseed/titiler) to serve local COG files would look something like:
@@ -131,6 +131,8 @@ using a local version of [titiler](https://github.com/developmentseed/titiler) t
 npm start -- --open --tileSourceTemplate="http://localhost:8000/cog/tiles/{z}/{x}/{y}?url={url}"
 ```
 
+If the option `useTileLayerAsFallback` is set to `true`, the tile server is only used as a fallback.
+
 ### buildTileUrlTemplate
 
 A more flexible option than `tileSourceTemplate` is passing a function to `buildTileUrlTemplate`.
@@ -140,12 +142,25 @@ This option replaces the v2 option `tileProxyUrl`.
 
 Please note that this option can only be provided through a config file and is not available via CLI.
 
+If the option `useTileLayerAsFallback` is set to `true`, the tile server is only used as a fallback.
+
 ### useTileLayerAsFallback
 
-If either `tileSourceTemplate` or `buildTileUrlTemplate` are given server-side rendering of COGs is enabled. 
+Depending on this option, either client-side or server-side rendering of imagery such as (cloud-optimized) GeoTiffs can be enabled/disabled.
+
+If either `tileSourceTemplate` or `buildTileUrlTemplate` are given server-side rendering of GeoTiffs is enabled. 
 If server-side rendering should only be used as a fallback for client-side rendering, enable the boolean `useTileLayerAsFallback` option.
 
-By default, client-side COG rendering is enabled. A server-side fallback is provided via the [tiles.rdnt.io](https://github.com/radiantearth/tiles.rdnt.io) project, which serves publicly accessible COGs as tile layers.
+To clarify the behavior, please have a look at the following table:
+
+| `useTileLayerAsFallback` | `tileSourceTemplate` / `buildTileUrlTemplate` | primary imagery renderer | fallback  imagery renderer |
+| ----- | ---------------------- | ----------- | ----------- |
+| true  | one of them configured | client-side | tile-server |
+| false | one of them configured | tile-server | none        |
+| true  | none configured        | client-side | none        |
+| false | none configured        | none        | none        |
+
+By default, client-side rendering is enabled. A server-side fallback is provided via the [tiles.rdnt.io](https://github.com/radiantearth/tiles.rdnt.io) project, which serves publicly accessible GeoTiffs as tile layers.
 
 ### displayGeoTiffByDefault