Image manipulation

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How can I get original reflectance data from the satellite?

To get original reflectance data from the satellite set the custom script accordingly. REFLECTANCE is a physical format, which requires a 32-bit TIFF float. The values ​​are from 0 to 1. See example below:

 https://services.sentinel-hub.com/ogc/wms/{INSTANCE_ID} ...&LAYERS=B04_REFLECTANCE&FORMAT=image/tiff;depth=32f

Custom script example for B04_REFLECTANCE:

return [B04]

Custom script band input values are already in reflectance (hence the simple script).

What is downsampling and upsampling?

Upsampling and downsampling define the method used for interpolation of the data on non-natural scales. E.g. resolution of Sentinel-2 data (R,G,B and NIR bands) is 10 meters but in some occasions you would want to look at the data with higher scale (e.g. at 1 m pixel resolution) or lower scale (e.g. 1000 m pixel resolution). 
A default option is "nearest neighbour", which is best for performance. "Bicubic" is often nicer  on higher scales.

If we do not specify the time range in the endpoint URL, can we assume that the images returned by the WMS service are the latest acquired?

Images are ordered by “Mosaic order” priority (see Configuration Utility, can be set for each layer; you can also set this in a parameter). You can choose either to have most recent on top or least cloud coverage. You should also take into account “maximum cloud coverage” parameter. 
In case you want to get the most recent images acquired, you should set maximum cloud coverage to 100% and priority to “most recent”

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