OT: what determines dropbox files not sync'd?

[joe at actionline.com]

What determines which dropbox files are accessible on a device without a
wireless connection and which are not accessible without a wireless
connection?

I have noticed that some text files that I copy to a folder in dropbox
are immediately available and some seem to only be available when the
device has a wireless connection.

Also, what is the best (most efficient) way to protect (secure) dropbox
files on a device from being accessed if the device is lost or stolen but
still have those files accessible in daily use?