Which part of an AWS CloudFormation template allows a customer to customize things within the stack by passing a value to a template at runtime?

In an AWS CloudFormation template, the section that allows customers to customize the configuration of the stack by passing values at runtime is the Parameters section.

Parameters act as inputs to the template and enable users to specify values when they launch a stack. This capability allows for greater flexibility, as the same template can be used to deploy multiple stacks with different configurations based on the values provided for the parameters. For example, a user might define a parameter for the instance type to deploy, allowing them to choose between different types when creating the stack.

Mappings, on the other hand, are used to create fixed configurations where specific keys are mapped to corresponding values, making them less dynamic than parameters. The Resources section defines the actual AWS resources that will be created, and Outputs are used to return values after stack creation, but they do not provide a means to input customizable values at runtime. Thus, the Parameters section is essential for customization and user input during the deployment of a CloudFormation stack.