A diagram is a visual expression of a requirement or a set of requirements. Often, these sets of requirements are documented as a picture to help represent or convey something. Diagrams are an effective way of simplifying potentially complex concepts to help a wider audience of stakeholders understand them. 

Diagrams are often included in requirements documentation, with some of the most popular diagrams being entity-relationship diagrams, process diagrams, and use case diagrams. Many other popular diagrams are used by business analysts and it is important to consider the purpose and audience of a diagram before choosing which one to use.

There are different categories where models are applied, depending on the category's main purpose and scope. Some model categories can include the following:

• People and roles 
• Rationale
• Activity flow
• Capability
• Data and information

Let's now discuss each model category and understand how to apply it by considering a real-world example:

• People and roles: This model category represents organizations, groups of people, and roles, as well as their relationships within an organization and with a solution. 

A real-world example could be: If the business analyst has to develop a people and roles model to show all the roles involved in a change management process or project. They may choose to develop a stakeholder map showing roles such as the key project team roles, the business stakeholders involved in the change, and any change implementation roles. The role of relationships in relation to the project will also be represented.

Techniques used to represent people and their roles include organizational modeling, roles and permissions matrices, stakeholder lists, maps, or personas.

• Rationale: This model category tends to represent the reasons for a change and answers the question of "why" in different ways.

Let's look at an example of a vehicle sales process:

Using the rational model category, a decision model can be used to illustrate the different decisions that must be made as part of the vehicle sales process. There will be multiple decision points within this process and each point will trigger a different flow. Some example decision points in the context of a vehicle sales process are as follows:

• Is the client choosing a new or used vehicle?
• Will the client pay with cash or choose a different finance option?
• Did the client accept the vehicle quotation price or was it declined?

There are a number of other modeling techniques that you can apply to depict the model category for showing a rationale; these include scope modeling, a business model canvas, root cause analysis, and business rules analysis.

• Activity flow: This model category represents a sequence of actions or events or a course of action that may be taken. 

Let's continue the real-world example of a vehicle sales process by applying a process model to represent the activities in the vehicle sales process. Some of the high-level activities can include the following: 

1. The salesperson receives a customer inquiry.
2. The customer indicates whether they want a new or used vehicle.
3. The salesperson suggests a potential vehicle to the customer, including a price for the vehicle.
4. The client accepts or declines the offer.
5. If they accept the offer to purchase, then the purchase contract will be produced and signed. However, if they decline the offer, the transaction is closed and the process ends.

All these activities or steps in the process will be documented with a visual activity flow model. There are a number of different techniques that can be used to represent activity flows. These include techniques such as process models, use cases and scenarios, and user stories.

• Capability: This model category is a visual representation of the features or functions of a solution or an organization.

For example, a capability model can be used to represent the different functions that an internet banking application should have. These functions may include the ability for a new user to sign up to the system, the ability to verify existing users, and the ability to enable password changes, as well as functionality to manage a bank account online.

Techniques that could be used to model the capabilities of a solution include business capability analysis, functional decomposition, and prototyping.

• Data and information: This model category represents the characteristics and the exchange of information within an organization or a solution. 

Techniques that are used to represent data and information include a data dictionary, data flow diagrams, data modeling, a glossary, state modeling, and interface analysis.