S3 as external vs S3 as primary

Server
1

If S3 is charged in enterprise as primary, all would prefer to go S3 as external with Community edition,
What is the difference between using S3 as a primary storage and S3 as a external storage ?

2

You would install ownCloud directly on S3, if you want to use S3 as primary storage. You can find the configuration here.
ownCloud has got an included external storage support. That's like a mount command of an external storage on your ownCloud server. You can execute it via GUI like here explained.