David says the signature type diagram is confusing, maybe it needs more explanation?

I suppose the (at least) 2x2 Matrix of types of component signatures is hard to make sense of, and hard to visualize.

book/source/06-signatures.md

@@ -28,15 +28,26 @@ For the purpose of this document, the term signature will refer to OpenPGP signa
 
 The OpenPGP standard defines a set of [Signature Types](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-signature-types), each identified by a numerical *signature type ID*. Signature types define the intent of a signature, and how it needs to be interpreted.
 
+Most OpenPGP signature types can be classified as either:
+
+- *Signatures over data* (type IDs `0x00` and `0x01`), or
+- *Signatures on components* (that is: signatures that apply to component keys or identity components).
+
+Signature on components are a complex topic, which we discuss in depth in {ref}`component_signatures_chapter`. They are grouped in two dimensions:
+
+- Who issued the signature (self-signature vs. third party signature)?
+- What kind of statement does the signature make (certify an identity, or bind component keys into a certificate)?
+
 ```{figure} mermaid/06-terminology.png
 
 An overview of signature types in OpenPGP
 ```
 
-Most OpenPGP signature types can be classified as either:
+```{admonition} TODO
+:class: warning
 
-- *Signatures over data*, or
-- *Signatures on components* (that is: signatures that apply to component keys or identity components).
+Group "Third-Party Certification" and "Self-Signature" as "Signature on components", in the diagram?
+```
 
 In this chapter, we discuss the general principles of OpenPGP signatures, which apply to all types of OpenPGP signatures.