As mentioned in my post on Modeling Languages, UML is one of the most popular and most used in software development. UML stands for Unified Modeling Language. It’s called Unified because it was a combination of several.

UML can be used in a variety of diagrams to express functionality.

• Secuence Diagrams
• Class Diagrams
• Object Diagrams

Sequence diagrams are utilized to describe the processes within a system. The communication between its components. If we are talking in software development, specifically in Object Oriented Programming, the communication would be messages sent between objects which would cause a behavioral change.

Class diagrams in UML are utilized to describe the name, attribute (values or data) and behavior (methods) of a class.

Finally, the object diagram is used to represent hierarchy ob objects when using inheritance.

To recap, depending on what is being represented, one diagram may be suitable over the other. If the customer wants to view functionality, creation or hierarchy within the system.

Sources:
https://www.lucidchart.com/pages/uml-sequence-diagram
https://www.ibm.com/developerworks/rational/library/3101.html
https://www.uml-diagrams.org/sequence-diagrams.html
https://www.visual-paradigm.com/guide/uml-unified-modeling-language/what-is-sequence-diagram/