The functions of sales document type are :

It can determine 
whether the sales doc of that type are blocked or not
No ranges for the sdocuments
whether a reference doc is mandatory or not
whether it should read cus.material info record
whether the system should run credit checks and how it 
should respond  etc..

A sales document type fullfills a requirement of a specific
business transaction.. for example in a cash sale you would
require the delivery to be happenign immediately after the
order and you would require a invoice slip to be handed over
to the customer immediately after his purchase..These things
are customised in your Cash sales(CS) document typr..

documents is  nothing but datacontainer,and its aproof of 
transactions 
to simply fy business process document categorised into 3 
types sales document
      delivery document      
      billking document
sales document :sales related data is going to be captured 
here to map differnet different process transactions sap 
has categotised into 4 types.

presales order :inq,qut
standard order:or,cash rush
customeroutline agreement:agreement&contracts
custemer complaints :credit,debit 
 next above what they given info is added

Different types of business processes are identified and 
controlled using sales document types.

the standard order for sales on fixed delivery date against 
invoice???

cash sale and rush order for sales from plant with or 
without invoice.

free of charge delivery for sales on fixed dlvry date 
without invoice. etc...