The flange is a ring that connects the garbage disposal to the sink and is slightly sloped to allow water to run unimpeded into the drain; it's like a mounting bracket for the disposal. If you are installing your sink with a waste disposal unit/garbage disposer, the disposal flange or trim will come with it and is typically in stainless steel. Here is a link to our collection of disposable flanges where you can find different brands and color options. Please check compatibility with your disposer.
An extended flange is required for sinks that are 3/4-inch to 1-3/4-inches thick. The extended collar is designed to fit fireclay applications and garbage disposals. Do check the product page of the sink to determine if you need an extension and if it is compatible with your disposer.