Why is there two exclamation points in a row at some explicit typing tags, while there's only one exclamation point at others?

[galaxia4Eva]

For Chapter 2 at spec

• - [ ] Clarify: Why is there two exclamation points in a row at some explicit typing tags, while there's only one exclamation point at others?

Example 2.23 Various Explicit Tags:

---
not-date: !!str 2002-04-28

picture: !!binary |
 R0lGODlhDAAMAIQAAP//9/X
 17unp5WZmZgAAAOfn515eXv
 Pz7Y6OjuDg4J+fn5OTk6enp
 56enmleECcgggoBADs=

application specific tag: !something |
 The semantics of the tag
 above may be different for
 different documents.

Example 2.24 Global Tags:

%TAG ! tag:clarkevans.com,2002:
--- !shape
# Use the ! handle for presenting
# tag:clarkevans.com,2002:circle
- !circle
  center: &ORIGIN {x: 73, y: 129}
  radius: 7
- !line
  start: *ORIGIN
  finish: { x: 89, y: 102 }
- !label
  start: *ORIGIN
  color: 0xFFEEBB
  text: Pretty vector drawing.