allow referencing existing lists as members.

63.md

@@ -35,6 +35,25 @@ The lists are constituted of one event for each _premium-reader_, and their remo
 }
 ```
 
+Besides marking individual public keys as readers it's also possible to tag a replaceable list, identified by its address:
+
+```yaml
+{
+  "kind": 1163,
+  "pubkey": "<content-creator>",
+  "tags": [
+    ["a", "<kind>:<pubkey>:<d-tag>"],
+  ],
+  // ...other fields
+}
+```
+
+This allows for an easy way to, for example, automatically mark all the people the _content-creator_ follows as allowed to read. Or people who are in a specific `kind:30000` follow-set.
+
+More importantly, it allows the _content-creator_ to delegate inclusion of readers to, for example, a payment provider, such that someone can pay and immediately become a _premium-reader_ without having to wait until the _content-creator_ is online again to update sign a new event.
+
+It remains a requirement that lists referenced in `"a"` tags here are sent directly to the relays that will implement the paywall, although such relays may try to fetch those in a best-effort basis.
+
 ### Relay behavior
 
 A relay that implements this NIP should: