Q: What is the difference between "Send Immediately" and "Send Automatically" with auto text messages an emails?
A: "Immediately" and "Automatically" are very different - the differences is this...
Immediately - sends the messages as soon as the activity is created (as soon as the workflow is started). This is normally reserved for the first email/text in a workflow.
Automatically - sends the message automatically based on the assigned date/time of the activity. In order for Automatic to work, you have to have it turned on in Settings, Emails, Autosend Message.
To use Immediate requires that you NOT use checklists, repeating activities or Multiple actions. Immediate is for basic activities which should not take a user's interaction to process.
To use Automatic requires that you NOT use checklists or Multiple actions. If you do have checklists or multiple actions, then automatic will not work "automatically". You will have to complete the activity and run the action manually.