1) there are several differences. External means exactly that. The files are located on a directory external to FileMaker. As I understand it, the secure option encrypts these files so that some one cannot access the folder directly in order to access the files that were originally inserted into container fields in your database.
2) use a container field "optimized for interactive content". This is a data format option that you select for a field while in layout mode. Then set up the users' privilege set(s) inside Manage | Security to not allow exporting data from the table where the container fields are defined.