When exporting BCS entities, you may have noticed 2 different "Settings" options available in the export dialog. If you're wondering what the differences are in the output - there don't appear to be any at all.
To confirm this, I just did an export of 2 entities to a model file using the "Default" option and the other export using the "Client" option. Using the Notepad++ text compare tool, there was no physical difference between the output of the different export settings - nor does there seem to be any documentation regarding this dropdown at all.
However, if you look at the BCS connection information, you'll also notice 2 tabs that match the different settings or "profiles" that you can choose. Consequently, this is expected behaviour - as you'll only have differences when your settings are different between default and client.