An attachment is a copy of the current content of an existing file duplicated into EfficientPIM, while a file link is only the record of the location of a file on the disk.
If the source file on the disk is modified, when you view an attachment file of the type 'File Link', you will see the latest information; on the other hand, if the type of the attachment is 'Attachment', you will see the information at the very time when it was attached.
Tip: File links can improve efficiency and performance. However, if your information is saved in a USB drive, or is to be shared among computers, you'd better use attachments.
