In Unity YAML, what does !u! mean?

For example, here’s an excerpt from a material:

%YAML 1.1
%TAG !u! tag:unity3d.com,2011:
--- !u!21 &2100000
Material:
  serializedVersion: 6
  m_ObjectHideFlags: 0
  m_PrefabParentObject: {fileID: 0}

I’m just trying to better understand some Unity internals.

I haven’t really worked much with YAML, but as you can read in the specs an exclamation point “!” specifies an explicit type in this context. Unity defines custom types based on the internal type nummer. You can find a list of the types here in the docs

So the first “!” tells you an explicit type identifier follows and Unity uses the following scheme for it’s custom types: “u!classID”

Also you may have a look at this documentation page