Sometimes you need to be able to collect information about your plans beyond just timelines and ownership - that's where custom fields can come in. Once you've configured some custom fields and saved them as templates - see Create Templates and Custom Fields, you can then start applying these templates to your plans to give you scalability and unlock some strategic insights that're more relevant to your organization!
Go to the desired plan, and in the Planner page, click the three dots against the Plan name, and select Plan Settings.
Expand the Custom Field Template field, select the desired template from the list and click Save.
This will apply the custom fields defined on that template to the corresponding entities on the plan. You can remove or update the template applied to a plan at any time from Plan Settings. Let's see how to populate and visualize data from custom fields in Populate Data against Custom Fields article!