Default logo is basic type of logo. Which will call in every header and every devices. So, first and foremost important logo format is default logo. For better viewing in all type of device you can upload 2x size of your logo and mention it's width and height.

Retina logo is only for retina devices. So, please upload 2x size from your default logo. You can also control this large logo width and height from our theme options. So, you'll not loss logo image quality in retina devices.

Default Small logo is basic type of logo shows in left sidebar header. Which will call in every left side header and every devices.

This header option will find under page options. Each page metabox you'll find header transparent option. That'll make header as transparent look. So, some of brand logos will not fit transparent background colors. In that time you take advantage of this feature.

Transparent Retina logo is only for retina devices for transparent header. So, please upload 2x size from your default logo. So, you'll not loss logo image quality in retina devices.

This is just a login page logo of your site admin. There is no more trick on it. If you not upload any logo in this area, you'll get default WordPress logo as it is. Login link will be like : http://yoursite.com/wp-login.php