When you create Requirements they are numbered by default with their unique IDs. The default Requirement IDs in the modeling tool are hierarchical numbers: 1, 1.1, 1.1.1, etc. See the following figure.
Requirements numbering.
The special validation rules ensure that each Requirement ID in your project is unique. For example, it is valuable when working with Teamwork Server or Teamwork Cloud and Requirements appear with the same IDs after commits. The validation rules check that and show errors.
If you want a different Requirement numbering than the default, you can customize it manually by using the Element Numbering dialog. However, it is not recommended.