diff --git a/ForumApp.markdown b/ForumApp.markdown
index 4c19e40..56b03d1 100644
--- a/ForumApp.markdown
+++ b/ForumApp.markdown
@@ -12,15 +12,15 @@ The application identifer is random:
 
 ### Message types
 
-**0: INVITATION** - The content is a BDF list with three elements: `forum` (list), `salt` (raw), and `note` (string, may be null). `forum` is a list with two elements, `name` and `salt`, as described above. The random salt in the invitation  `note` is an optional note from the inviter to the invitee.
+**0: INVITATION** - The content is a BDF list with three elements: `forum` (list) and `note` (string, may be null). `forum` is a list with two elements, `name` and `salt`, as described above. `note` is an optional note from the inviter to the invitee.
 
-**1: RESPONSE** - The content is a BDF list with three elements: `invitation_id` (raw, HASH_LEN bytes), `decision` (boolean), and `note` (string, may be null). `invitation_id` is the identifier of the INVITATION to which this is a response. `decision` indicates whether the invitee wishes to join the forum. `note` is an optional note from the invitee to the inviter.
+**1: RESPONSE** - The content is a BDF list with three elements: `invitation_id` (raw), `decision` (boolean), and `note` (string, may be null). `invitation_id` is the identifier of an invitation created by the opposite device. `decision` indicates whether the invitee wishes to join the forum. `note` is an optional note from the invitee to the inviter.
 
-**2: DEPARTURE** - The content 
+**2: DEPARTURE** - The content is a BDF list with one element: `invitation_id` (raw), which is the identifier of an invitation created by either device.
 
 **3: POST** - The content is a BDF list with two elements: `author` (list, may be null) and `content` (list). If `author` is null, the post is anonymous. Otherwise `author` is a list with two elements: `identity` (list) and `signature` (raw). `identity` is a list with two elements: `name` (string) and `public_key` (raw). `signature` is calculated over `content`.
 
-`content` is a list with three elements: `parent_id` (raw, HASH_LEN bytes, may be null), `body` (string), and `attachments` (dictionary, may be null). If `parent_id` is not null, it is the identifier of a POST to which this is a response. Each key in `attachments` is the name of an attachment, and the value is a list with two elements: `mime_type` (string) and `message_id` (raw, HASH_LEN bytes).
+`content` is a list with three elements: `parent_id` (raw, may be null), `body` (string), and `attachments` (dictionary, may be null). If `parent_id` is not null, it is the identifier of a POST to which this is a response. Each key in `attachments` is the name of an attachment, and the value is a list with two elements: `mime_type` (string) and `message_id` (raw).
 
 **4: ATTACHMENT** - The content is raw data.
 
@@ -29,7 +29,8 @@ The application identifer is random:
 * An invitation or response must belong to the invitation channel.
 * A post or attachment must belong to a forum channel.
 * An invitation is valid if it is well-formed.
-* A response is valid if it is well-formed and it references a valid invitation created by the opposite (local or remote) device.
+* A response is valid if it is well-formed and references a valid invitation created by the opposite device.
+* A departure is valid if it is well-formed and references a valid invitation created by either device.
 * A post is valid if it is well-formed, its parent (if any) is a valid post, and it is either anonymous or carries a valid signature over its content.
 * An attachment is always valid.
 
@@ -37,15 +38,20 @@ Note that a post can be validated before its attachments have been received, and
 
 ### Storage policy
 
-* All valid local messages are stored.
-* All valid remote messages are stored.
+For the invitation channel:
+* All local messages are stored.
+* All remote messages are stored.
+
+For forum channels:
+* Local messages are stored while the local device belongs to the forum.
+* Remote messages are stored while the local device belongs to the forum.
 
 ### Sharing policy
 
 For the invitation channel:
-* A valid local message is shared with one remote device.
+* Each local message is shared with one remote device.
 * Remote messages are not shared with other remote devices.
 
 For forum channels:
-* Valid local messages are shared with all remote devices that belong to the forum.
-* Valid remote messages are shared with all remote devices that belong to the forum.
+* Local messages are shared with all remote devices that belong to the forum.
+* Remote messages are shared with all remote devices that belong to the forum.