I'm using the FOSS version of Zimbra 7.2.3 on Ubuntu.

Normally, we don't use calendar and contacts synching, or even shared calendars.

I have just recently had a request to start using shared calendars, free/busy time so that users can schedule meetings with other users.
Additionally, I have a user who would like to synch their contacts with their iPhone.

I have found ZPUSH plugin, which is found here: Z-Push Zimbra Backend | Free software downloads at SourceForge.net
I have also found that Zimbra FOSS version supports CalDAV and CardDAV (found here: https://www.zimbra.com/products/compare_products.html)

What is the difference between what the FOSS version supports out of the box and an additional plugin such as ZPUSH or even Zextras (found here ZeXtras SUITE for your Zimbra Server).

Thanks for any explanation of the differences.