There are four basic types of call methods you can configure in a use case.
CODE
This is when a desktop or mobile agent generates a URL to send to a guest to connect. This url contains a call url and a pin code.
For more information, refer to Creating a CODE Use case.
ACD call method
ACD means Automatic Call Distribution, these are calls initiated by a guest, that are then routed to an Agent. Usually they will click on a pre-existing URL to connect to certain "type" of agent (based on skill or location routing).
For more information, refer to Creating an ACD use case.
Mobile
Similarly to the CODE call method, except specifically designed for Mobile Agents.
For more information, refer to Creating a Mobile Use case.
Direct
(Deprecated) Call an agent directly