diff --git a/book/source/04-certificates.md b/book/source/04-certificates.md
index 7790c74..841a49e 100644
--- a/book/source/04-certificates.md
+++ b/book/source/04-certificates.md
@@ -522,9 +522,11 @@ The next part of this packet contains hashed subpacket data. A subpacket data se
 
 There are two sets of subpacket data in a Signature: hashed, and unhashed. The difference is that the hashed subpackets are protected by the digital signature of this packet, while the unhashed subpackets are not.
 
-The following subpacket data consists of sets of "subpacket length, subpacket tag, data." We'll show the information for each subpacket as one line, starting with the [subpacket type description](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-signature-subpacket-specifi) (based on the subpacket tag). Note that bit 7 of the subpacket tag signals if that subpacket is ["critical"](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#section-5.2.3.7-10)[^critical].
+The following subpacket data consists of sets of "subpacket length, subpacket tag, data." We'll show the information for each subpacket as one line, starting with the [subpacket type description](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-signature-subpacket-specifi) (based on the subpacket tag). Note that bit 7 of the subpacket tag signals if that subpacket is ["critical"](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#section-5.2.3.7-10).
 
-[^critical]: Critical here means: the receiver must be able to interpret the subpacket and is expected to fail, otherwise. Non-critical subpackets may be ignored by the receiver
+```{note}
+Critical here means: the receiver must be able to interpret the subpacket and is expected to fail, otherwise. Non-critical subpackets may be ignored by the receiver.
+```
 
 - [Signature creation time](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#signature-creation-subpacket) (subpacket type 2) *critical*: `0x6516eaa6` (also see [Time Fields](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-time-fields))
 - [Key expiration time](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#key-expiration-subpacket) (subpacket type 9) *critical*: `0x05a48fbd` (also see [Time Fields](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-time-fields))