The product public library, at account.products.product.public, contains resources such as files that are required by the product and which may need to be served to users who have not yet installed the product, such as product information pages.
See Public in the sample product for an example of a public folder.
At the very least, the public library should contain a product icon as a 64x64 pixel png, with a local reference of icon.
Files required by the product, such as icon files, may be held in the public folder if convenient, and if there are no security implications for doing so. This can be useful if the files are referenced in the documentation, for example to describe the icons used in the product.
Files required for the product theme should not be held in the public folder, but should be held in a separate theme folder.