On the newer enhanced plc-5's the processor generates a checksum everytime the program is saved. So if Bill goes on line and saves the file a checksum with the value of x is saved with the file. Then Bob comes along and goes on line the saved file on his pc has a checksum with the value y. Since y does not equal x, you have to preform an upload to ensure you have the latest logic, that's actually running in the plc. Now in my previous example, if Bill went back on line before Bob did, the checksums would match and no upload would be required.
Make sense, clear as mud?
On the "classic" platforms, you don't have a choice, you always have to an upload. I believe this is because they lack the checksum feature.
So if you have a 5/10, 5/15 or 5/25's at work you must always do the upload.