Basic Flow is the flow of the main events (from the time of log in until exit), that defines the interactions between the actors and the system. It is a flow that helps the actor achieve a specific goal as per the plan for the particular use case.
Example: A passenger buying a ticket
The passenger logs into the system with his user name and password.
System accepts the log in details.
The system asks for details of travel.
The passenger keys in the details.
The system asks for payment details.
The passenger provides the card details.
The system approves the transaction.
The ticket is reserved and the print collected by the passenger.
The use case ends.
Alternate Flow : The flow of events are disrupted and the action is not complete or the goal is not achieved.
The customer/passenger keys in incorrect payment details.
The system prompts an error message.
The passenger re-enters the card details.
The system shows or displays a message that the account balance is insufficient.
The passenger tries one more time.
The system pops up a message that the transaction is not approved as there is an issue with the banks server, to try later.