Setting default value of archiving level for message on a server

Setting this property will change default archiving level for messages for every account on server for which per account default archiving level is not set. User will be able to change this value setting default modes as described in XEP-0136 section 2.4

Possible values are:

Messages are not archived
Only message body will be stored. Message without a body will not be stored with this value set
While message stanza will be archived (if message should be stored, see the section called “Saving Options”)
In this mode every stanza should be archived. (Not supported)

To set default level to message you need to set default-store-method of message-archive processor to message:

sess-man {
    message-archive {
        default-store-method = 'message'