A lot of the structures used in Vulkan have a sType member, which identifies the type of the structure, and a pNext member, for extension-specific structures. This answer explains quite well what the sType member is for, and why it is needed. It briefly touches on pNext, although I'm not sure I understand the rationale behind it.
If the first member of every structure is sType, couldn't extensions just define their own structure types when they need different/extra parameters?