What can be defined in sites-enable/a_site

[James Sumners]

It seems that in addition to virtual server blocks I can also define 
global clients in a site file. I'd really like to contain all of the 
configuration specific to my site in one file, including modules. But 
it doesn't seem like I can define global modules in a site file. Is 
this correct? I can't find any description of what is actually allowed 
in such a file; just a description of what is allowed in a virtual 
server block.