Original vs. thumbnail

When I ran Thumbnail Cleaner, it took out some original files. When I replaced the backup made with the plugin, one of them came back; the other was still gone.

The files taken out were named with dimensions in the original file. Could you tell me exactly what TC looks for to identify a thumbnail? Is it any 2 series of digits with a ‘x’ between? So if I name them *_2000-701.jpg, it won’t be identified as a thumbnail? Or if there are other characters after the dimensions, like *_2000x701cc.jpg?

Thanks