The link between schedule types and credentials happens when you add roles to your schedule type because credentials are attached to roles.  See below for an example of an employee roles listing page.  

Notice the Lifeguard role has 4 credentials staff are required to have to fulfill the role of lifeguard.  

Below is an example of a schedule type listing page.  Roles are added to the schedule type.  The schedule type AQUA has Lifeguard as a role and the role Lifeguard, shown above, lets you know the credentials required for this role.