It is Software Design Principles, as below

• S - Single-Responcibility
• O - Open-closed
• L - Liskov Substitution
• I - Interface segretation
• I - Dependency inversion

Single-Responcibility - Class or entities should have one only task and only reason to change that particular task, meaning do not define all the functionalities in the single class/enity.
Open-closed - Class or entities should allow to extend further but not modification. So Open for extension and closed for any modification.
Liskov Substitution - Subclasses should be substitutable for base classes.

When we define class and no intention to inherit from any other class and they can never be used as a base class. The sealed modifier is used to prevent derivation from a class. We encounter compile-time errors if a sealed class is specified as the base class of another class. A sealed class cannot also be an abstract class. .Net framework cliams that, some run-time optimizations can make calling sealed class members slightly faster.