What do the various uses for assets mean?

Site - It will appear for the assigned site.  The assets will appear on the client site on the assets page.  You will need a link to the assets page in the navigation of the site.  In a B2B site, Assets is one of the standard menu items.  

Group - It will appear for the assigned groups.  For example, if a group contains multiple users, it will appear for all of those users.  The assets will appear on the client site on the assets page.  You will need a link to the assets page in the navigation of the site.  In a B2B site, Assets is one of the standard menu items.  It will also appear as an option for image fields on eDoc Builder standard templates.

User- It will appear for the assigned user.  The assets will appear on the client site on the assets page.  You will need a link to the assets page in the navigation of the site.  In a B2B site, Assets is one of the standard menu items.  It will also appear as an option for image fields on eDoc Builder standard templates. 

Product - It will appear for the assigned product on the product page if "Customer Can Download" is enabled in Sites > [site name] > Assets > [asset name].  It will also appear as an option for image fields on eDoc Builder standard templates.  When the product is ordered, the asset will appear in the Files tab of the of the order in the Orders screen in admin.