“Observers of the kernel’s commit stream or mailing lists will have seen a certain amount of traffic referring to the addition of SPDX license identifiers to kernel source files. For many, this may be their first encounter with SPDX. But the SPDX effort has been going on for some years; this article describes SPDX, along with why and how the kernel community intends to use it. ”
For more see the article by Jonathan Corbet of LWN “SPDX Identifiers in the Kernel”.