Each solid line represents an interaction between an actor and the system
Each dotted line represents an interaction between a use case and another use case

Actors

Actors are the objects that interact with the use cases.
They can be either initiators or participants

Interactions

<<initiates>>

Usage: For actors that call a use case Syntax: Solid line between actor and the use case

<<participates>>

Usage: For actors that are involved in a use case Syntax: Solid line between actor and the use case

<<includes>>

Usage: For required use cases
Syntax: Dotted line FROM the use case, TO the use case that is included

<<extends>>

Usage: For optional use cases
Syntax: Dotted line FROM the extension, TO the main functionality

Dotted line (with arrow) from primary to secondary case