.ASPX is the extension for Microsoft's .Net framework web application files. They are a dynamic scripting page.
.HTML and .HTM indicate static pages.
Currently search engines don't rank based on extension. If you prefer to have .html extensions, this can be changed by following Stores > Active Store > Store Settings and then Select SEO Settings from the left handside navigation and enable .html extension
AmeriCommerce uses dynamic pages by default, but you can easily switch to .html instead of .aspx.