URL component aliases are useful when you update URL components that point to products on your site, and you want to keep a list of the URL aliases that were used in the past. By enabling the URL Component Aliases feature, you can consolidate all your different variations into one main URL component and avoid duplicate content.
To enable the URL Components Alias feature:
Go to Setup > Company > Enable Features.
Click the Web Presence subtab.
Check the box next to URL Component Aliases.