The task Schedule tab is where you estimate hours and schedule resources to tasks. Resources can be scheduled without a role if role-based cost and revenue estimates are not required. See Creating Task Time Estimates.
When roles are not used, the result is an estimate for total task hours, and a schedule for each resource's hours on the task.
When roles are used, you get estimated cost and revenue values in addition to the total estimated hours and resource scheduled hours. In order to schedule a resource to a role on a task, the resource must be allocated to that role on the project.
You can make project-level financial estimates, if desired. See About Financial Entries for more information.