In a non-SXA website, you have created a layout definition item and want to designate...

According to Sitecore best practices, presentation details should be set on the standard values of the template that defines the item. This way, all items that are based on the same template will inherit the same presentation details, unless they are explicitly overridden. Setting presentation details on the standard values also allows resetting modified item presentation settings to default values. Setting presentation details on the individual items is not recommended, as it increases the maintenance cost and complexity of the presentation layer. Rendering parameters are used to control how renderings function, not to designate layouts. Standard template is a base template that all templates inherit from, and it does not have any presentation details. References:

Presentation Component Reference - Sitecore

Sitecore Presentation Layer Best Practices - Sitecore Developer Blog