diff --git a/25.md b/25.md
index 73b724c..0fb2af1 100644
--- a/25.md
+++ b/25.md
@@ -27,7 +27,8 @@ The last `p` tag MUST be the `pubkey` of the event being reacted to.
 
 The `a` tag MUST contain the coordinates (`kind:pubkey:d-tag`) of the replaceable being reacted to.
 
-The reaction event MAY include a `k` tag with the stringified kind number of the reacted event as its value.
+The reaction event SHOULD include a `k` tag with the stringified kind number of the reacted event as
+its value, and a `c` tag with the reaction's `content` so that reactions can be indexed and filtered.
 
 Example code
 
diff --git a/README.md b/README.md
index e03cb3b..24826df 100644
--- a/README.md
+++ b/README.md
@@ -251,6 +251,7 @@ They exist to document what may be implemented by [Nostr](https://github.com/nos
 
 | name              | value                                | other parameters                | NIP                                   |
 | ----------------- | ------------------------------------ | ------------------------------- | ------------------------------------- |
+| `c`               | category                             |                                 | [25](25.md)                           |
 | `e`               | event id (hex)                       | relay URL, marker, pubkey (hex) | [01](01.md), [10](10.md)              |
 | `p`               | pubkey (hex)                         | relay URL, petname              | [01](01.md), [02](02.md)              |
 | `a`               | coordinates to an event              | relay URL                       | [01](01.md)                           |