User stories are a technique used in business analysis to represent a small, concise statement of functionality or quality needed to deliver value to a specific stakeholder. They are expressed by presenting the role, the goal, or the value and are a fundamental component of Agile methodologies. User stories help teams understand what the stakeholders need without going into excessive detail, allowing for iterative development and refinement of requirements.
References: The information is verified as per the Business Analysis Learning documents and resources, which discuss various techniques including user stories as a way to capture stakeholder needs and prioritize development of solutions12.

Stakeholders often face challenges in prioritizing requirements due to different perceived values. This challenge arises because stakeholders may have varying opinions on what is most important, leading to conflicts and difficulties in reaching a consensus on priority. The diversity in stakeholders' backgrounds, roles, and interests can result in a wide range of perspectives on which requirements should take precedence.

There are two commonly used relationships between use cases:
Extend: allows for the insertion of additional behavior into a use case. The use case that is being extended must be completely functional in its own right and must not depend on the extending use case for its successful execution. This relationship may be used to show that an alternate flow has been added to an existing use case (representing new requirements).
Include: allows for the use case to make use of functionality present in another use case. The included use case does not need to be a complete use case in its own right if it is not directly triggered by an actor. This relationship is most often used either when some shared functionality is required by several use cases or to abstract out a complex piece of logic.

Decision Modelling and Estimation are techniques that can be applied to prioritize requirements effectively. Decision modelling helps in understanding the implications of various choices and selecting the best course of action based on the analysis. Estimation provides insights into the effort and resources required to fulfill the requirements, which is crucial for prioritization, especially when resources are limited or when there are time constraints.