Is a high level of formality and detail required?

Always remember that the more the level of formality increases, the more difficult it becomes to effectively trace a requirement.

There might be a few reasons why a certain level of detail is being documented for a specific requirement. 

Let's consider the following example to explain this further:

The requirement is to implement a new product within a bank's credit card division.

A high level of formality and detail will likely be chosen, as there will be many aspects, business rules, and other business area impacts to consider relating to this requirement.

For example, some of the complexities with this example can include product-specific business rules, product features, customer application rules, and customer verification processes. Even physical card designs and delivery processes will possibly need to be considered for this requirement. In this case, the detailed requirements will be traced formally and at a low level of detail.

Here's a second example:

The same credit card division has a requirement to be able to send their customers a text message once a new card has been issued for their account. Due to the simpler nature and size of this requirement as well as the smaller overall business impact, the level of formality and hence level of detail to trace the requirement to will be a lot lower than in the first example.