What's the difference between a regular and extended license?

This should be purchased by the owner of the website that the plugin will be installed on

Extended License: for use in a single end product for sale.

This license can be sold and transferred to another owner as part of an end product (the regular license must be purchased by the site owner and not resold as part of another project).

For example, this license is needed if you are a developer or agency that wishes to sell a site to their client as part of their product/service that includes auto-ship as the end product or service.  That way the license ownership can change hands and the original licensee can legally profit off of the sale of the license