Form patterns provide form structure, based on a particular style (including required and optional controls), and also provide many default control properties. Form patterns need to be specified when designing a form element. The reason why it is mandatory is because form patterns help guarantee that forms have a responsive layout. Finally, patterns also help guarantee better compatibility with upgrades. If your user interface design does not correspond to a given form pattern, you can select a custom form pattern; however, this means that you should test the form for a responsive layout on different devices, browsers, and form factors.

There are several form patterns and sub-patterns to choose from. Visit the following web page to find out more about form patterns and sub-patterns: https://docs.microsoft.com/en-us/dynamics365/operations/dev-itpro/user-interface/user-interface-development-home-page.