谈到追踪，CMMI中分了两类帮助我们理解不同的追踪关系：纵向追踪和横向追踪。
• 纵向追踪：Vertical Traceability
– 不同层次需求（信息）之间的追踪关系
– 保证用户的需求都被完整的设计/测试
• 横向追踪: Horizontal Traceability
– 同一信息层次上工作组/产品构件之间的关系
– (在集成测试之前)发现潜在的问题
这样的分类能够帮助我们迅速的理解整个项目信息之间的关系，但随着工作深入，有这样的一个想法，这样的说法是不是太过简单和绝对呢了？
首先呢，所谓Vertical和Horizontal，取决于观察者所立的角度，取决于对不同信息模块的布局。简单点说，纵向和横向表示物理空间的名词，没有具体语义。
其次，除了Vertical和Horizontal，有没有其他类型的关联类型。是不是非黑即白，或者说用这两种类型是否能够足够清楚的刻画信息追踪的关联关系。
需求关联的类型实际上反映了需求之间的关系类型，我的建议是不要简单的用横向或者纵向来区分。大家可以参见下面的一些在实际工作中需要重视的关系。在我看来，无所谓它（们）是属于横向或者纵向，关键是你需要清楚的知道你的项目中有哪些重要的信息模块，这些信息模块对其他信息的影响是如何的，它们的逻辑语义是什么。所谓牵一发而动全身，您的项目里能够很好的保证需求的一致性和变更的统一性吗？关键还是在于您能否清楚认识、建立和管理了下面的关联关系。
譬如说常见的关系类型
– 满足关系：系统规格满足用户需求
– 验证关系：系统测试案例验证系统需求
• 实际的应用中还有
– 依赖：软件功能依赖于硬件的选择
– 减轻(风险)： 需求－风险列表的关系
– 遵循：遵循某个重要的标准
原文的描述如下（摘自http://www.sei.cmu.edu/cmmi/faq/term-faq.html）：
In the Requirements Management (REQM) process area, specific practice 1.4 states, “Maintain bidirectional traceability among the requirements and the project plans and work products.” Bidirectional traceability primarily applies to vertical traceability and at a minimum needs to be implemented both forward and backward (i.e., from requirements to end products and from end product back to requirements).
Vertical traceability identifies the origin of items (e.g., customer needs) and follows these same items as they travel through the hierarchy of the Work Breakdown Structure to the project teams and eventually to the customer. When the requirements are managed well, traceability can be established from the source requirement to its lower level requirements and from the lower level requirements back to their source. Such bidirectional traceability helps determine that all source requirements have been completely addressed and that all lower level requirements can be traced to a valid source.
Horizontal traceability is also important and is mentioned in subpractice 3, but it is not required to satisfy bidirectional traceability. Horizontal traceability identifies the relationships among related items across work groups or product components for the purpose of avoiding potential conflicts. It enables the project to anticipate potential problems (and mitigate or solve them) before integration testing. For example, horizontal traceability would follow related requirements across two work groups working on two associated components of a product. The traceability across these two work groups enables the work groups to see when and how a change in a requirement for one of the components may affect the other component. Thus, horizontal traceability enables the project to anticipate potential problems (and mitigate or solve them) before integration testing.
