What is a BizTalk `application`

The Standard edition of BizTalk Server 2004 is restricted to "10 internal applications with up to 20 external trading partners".   A colleague just asked what the definition of an `application` is

I believe the answer is as follows.   An `application` in BizTalk is a synonym for a host.  The term `Application` is used in relation to the MessageBox, whereas the term `Host` is used in management and administration of BizTalk.   Note that default name for the default in-proc host is BizTalkServerApplication. 

The MessageBox maintains an `Application` table and BizTalk dynamically creates multiple queue tables in the MessageBox database for each application (i.e., each application has its own independent set of queues).   

When administering BizTalk, hosts are, of course, virtual concepts and can map to multiple host instances on different boxes.   However, the Standard version does not support multi-box clusters.   You can only have one instance of any on host on any one box.