MIGx Tutorial - Resource-specific Media Source

I'm using the Using resource-specific mediasource and multifile-uploader with MIGX tutorial.

In the media source, we're told to add three new settings:

thumbX: 200
thumbY: 200
maxFiles: 20

My question is, how are the thumbX and thumbY properties used?

they are used to generate the thumbs, seen in the uploader.

https://github.com/Bruno17/MIGX/blob/master/core/components/migx/processors/mgr/default/imageupload.php#L245

with another media-source-setting 'resizeConfigs'
you can define multiple file-sizes with a json-string,
I think it was like that: