Import process

Can someone please explain to me what tools/import xml files is doing in detail?

I can’t understand the difference between importing the file and checking ‘Download and import file attachments’ and importing the file and NOT checking this box.

The import for me works if I don’t check this box but does if I do (i have been told this is a size issue). But when i look at the import all the blog pictures seem to be there. But certain images in the theme (logo, homepage slider) are missing.

Please help me understand what is going on.

thank you