Sequence Diagram learning step by step

Sequence Diagram:

i) SD shows the interactions between objects in the sequential order.
ii) During the requirements phase of a project, analysts can take use cases to the next level by providing a more formal level of refinement. When that occurs, use cases are often refined into one or more sequence diagrams.
iii) how a future system should behave is documented using SD.
iv)

Why SD is required?

One of the primary uses of sequence diagrams is in the transition from requirements expressed as use cases to the next and more formal level of refinement. Use cases are often refined into one or more sequence diagrams.

In addition to their use in designing new systems, sequence diagrams can be used to document how objects in an existing (call it "legacy") system currently interact. This documentation is very useful when transitioning a system to another person or organization.

What is Notation?

i) Notation is a frame for drawing.
ii) The frame element is used as a basis for many other diagram elements in UML 2, but the first place most people will encounter a frame element is as the graphical boundary of a diagram.
iii) A frame element provides a consistent place for a diagram's label, while providing a graphical boundary for the diagram.

What is Namebox for a notation?

The diagram's label is placed in the top left corner in what I'll call the frame's "namebox," a sort of dog-eared rectangle.

What is "Interaction"?

On sequence diagrams incoming and outgoing messages (a.k.a. interactions) for a sequence can be modeled by connecting the messages to the border of the frame element.

How to give name for sequence diagram?

Diagram Type | Diagram Name

specific text values for diagram types
sd = Sequence Diagram, activity = Activity Diagram, and use case = Use Case Diagram

Main Purpose of sequence diagram:

i) The main purpose of a sequence diagram is to define event sequences that result in some desired outcome.
ii) The focus is less on messages themselves and more on the order in which messages occur; nevertheless, most sequence diagrams will communicate what messages are sent between a system's objects as well as the order in which they occur.
iii) The diagram conveys this information along the horizontal and vertical dimensions: the vertical dimension shows, top down, the time sequence of messages/calls as they occur, and the horizontal dimension shows, left to right, the object instances that the messages are sent to.

What is lifeline?

When drawing a sequence diagram, lifeline notation elements are placed across the top of the diagram. Lifelines represent either roles or object instances that participate in the sequence being modeled.
Lifelines are drawn as a box with a dashed line descending from the center of the bottom edge (Figure 3). The lifeline's name is placed inside the box.
An example of the Student class used in a lifeline whose instance name is freshman
The UML standard for naming a lifeline follows the format of:
Instance Name : Class Name

Asynchronus Message:

 An asynchronous message is drawn similar to a synchronous one, but the message's line is drawn with a stick arrowhead.

Resource Link:

1. https://www.ibm.com/developerworks/rational/library/3101.html

1. https://www.ibm.com/developerworks/rational/library/3101-pdf.pdf