A forum search would help find most answers to your many questions.
Use Advanced Search To Find Answers
Also the Enterprise knowledgebase HERE
(change country at top right if necessary)
Try the explanation here: Re: DAT Update Process
DAT's are required to add new malware discoveries to the software database.
The Consumer version of the software has dropped DAT updates in favour of silent Engine updates and VSE may follow suit at some stage, but not sure of that.
Because there are 2 million new viruses a day.