Sequence Diagram Notation Elements

(1) Dotted vertical line below an object/actor,The lifeline — shows the existence of that object/actor over time; you read the sequence top to bottom

(2) Thin rectangle on a dotted lifeline,An activation box — the period during which that object instance is involved in the computation

(3) Solid horizontal arrow between lifelines,A synchronous call or message sent from one object to another

(4) Dashed horizontal arrow between lifelines,A return value or response message

(5) [condition] on an arrow or inside an alt box,A guard condition — the interaction only occurs if the condition is true

Explanation

Sequence diagram notation: Dotted vertical lines = lifelines (objects/actors exist over time; read top to bottom); Thin rectangles on lifelines = activation boxes (object is actively involved); Solid arrows = messages/calls sent between objects; Dashed arrows = return values or responses; [condition] = guards specifying when an interaction occurs.

Symbol	Notation Title	Meaning & Description
	Dotted vertical lines	Represents lifelines, indicating that objects or actors exist over a period of time. These lines are read sequentially from top to bottom.
	Thin rectangles on lifelines	Represents activation boxes, which visually demonstrate the period during which an object is actively involved in an operation or processing a request.
	Solid arrows	Represents messages or calls sent synchronously between distinct lifelines/objects.
	Dashed arrows	Represents return values or responses sent back to the requester from a previous message activation.
[condition]	Guard conditions	Acts as guards, specifying evaluation criteria or conditional logic that must be true for that specific interaction or message exchange to take place.