Arguably, thumbnails is a topic for WP rather than FRT but I, as a novice, would be grateful to read here why FRT is needed and which image files it targets.
To begin at the beginning: what is a thumbnail? Is it the preview image which is displayed in the Image Library and in the Image Editor and in the Page Editor/Add Media window? Or is it also the different sizes of the uploaded image which are automatically created by WP or by responsive themes?
And does FRT delete all sizes of image which haven’t yet been added to a page or post? If yes then it is doing much more than regenerating thumbnails. For example, if I upload an image today but won’t be placing it in a page until tomorrow, will FRT remove it if I run it today?
And does FRT do anything as soon as I activate it or does it wait until I instruct it to scan?
And are there any special considerations for Retina-capable themes or does FRT treat them just the same as non-retina themes?
I’m sorry – this is very basic but I’m trying to understand the scope of FRT.