Each source code file needs to mention which license applies and who owns the copyright in the file header. The purpose of this is to make it absolutely clear, for both humans and tooling, which license applies to that particular file even when the file is moved to another project. In order to achieve what is stated above, the header should include:
A copyright statement
A license header
An SPDX tag
Examples of this setup for different programming languages can be found here: here
The rationale behind using both a license header and an SPDX tag is to make it readable for humans that do not know the abbreviations of licenses as well as making it explicit enough to support compliance tooling in a future proof way. As SPDX is the licensing standard that compliance tooling will be using in the future, and indeed already is used by tool makers such as WindRiver and Black Duck, they are included both for support for tooling and future proofing the format.