diff --git a/ForumApp.markdown b/ForumApp.markdown
index 5c4423b..4c19e40 100644
--- a/ForumApp.markdown
+++ b/ForumApp.markdown
@@ -5,39 +5,40 @@ ForumApp is a [BSP application](BSP) that synchronises forum posts among groups
 ForumApp has a single invitation channel with the following random identifier:   
 `667e 2acb da80 2e61 3168 98f2 8ec5 8102 fcf2 3d10 04ee 3086 f5e9 94d8 30f5 b773`
 
-Each forum has its own channel with a [hash identifier](BSP#channel-identifiers). The application data structure for generating the identifier is a [BDF](BDF) list with two elements: `name` (string) and `salt` (raw, HASH_LEN bytes). The random salt prevents collisions between forums with the same name.
+Each forum has its own channel with a [hash identifier](BSP#channel-identifiers). The application data structure for generating the identifier is a [BDF](BDF) list with two elements: `name` (string) and `salt` (raw). The random salt prevents collisions between forums with the same name.
 
 The application identifer is random:   
 `859a 7be5 0dca 035b 64bd 6902 fb79 7097 795a f837 abbf 8c16 d750 b3c2 ccc1 86ea`
 
 ### Message types
 
-**0: INVITATION** - The content is a BDF list with two 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.
+**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.
 
 **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.
 
-**2: 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`.
+**2: DEPARTURE** - The content 
+
+**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).
 
-**3: ATTACHMENT** - The content is raw data.
+**4: ATTACHMENT** - The content is raw data.
 
-### Validation
+### Validity policy
 
-* 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 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.
-* An ATTACHMENT is valid if it is referenced by a valid POST.
+* 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 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.
 
-Note that a POST can be validated before its attachments have been received.
+Note that a post can be validated before its attachments have been received, and an attachment can be validated before it has been completely received.
 
 ### Storage policy
 
 * All valid local messages are stored.
 * All valid remote messages are stored.
-* An ATTACHMENT that is not referenced by a valid POST may be garbage-collected.
 
 ### Sharing policy