One-Way
All 3rd party calendar communication is one-way. ABPro can insert/remove data from the 3rd party calendars but they have no knowledge of ABPro. Changes made in 3rd party calendars have no effect in ABPro.

Do I need a 3rd party calendar?
No!

I added support for a 3rd party calendars in response to users wanting a ‘calendar’ view of bookings for their staff, or they were already using the 3rd party calendar, for something else, and wanted to integrate ABPro.

I added the ABPro Front-Desk View so customers could get a ‘calendar’ view built specifically for ABPro. It has the ability to drill down to booking details, no 3rd party calendars can do that.

Technical stuff
ABPro’s communication with 3rd party calendars is strictly one-way. ABPro adds/removes bookings by directly accessing the calendar’s database tables. No 3rd party calendar has any knowledge of ABPro and so any changes made in the 3rd party calendar will have NO effect on ABPro.

JEvents and JCalPro used to be pretty much the same as far as functionality.
JCalPro looked much better but was not free.
JEvents was(is) free.
EventList is not a calendar at all but people using it wanted integration with ABPro.
Thyme is a cool calendar but not free and not a Joomla component. It has tools to allow it to be used in Joomla but it has a steep learning curve.
Google Calendar is my favourite. Easy to setup and use. A nice feature of Google is its ability to sync with Outlook’s calendar.

With the latest versions;
JCalPro enhanced the product in several ways. ABPro now supports either JCalPro or JCalPro2
JEvents did a total re-write moving to iCal design and ABPro does NOT support the new version. The way ABPro talks to 3rd party calendars is by inserting and removing data from tables. The iCal data model is very complex and the JEvents authors have told me that accessing the data tables directly would be very difficult. I have taken them at their word and dropped support for JEVents. JEvents 1.5 is not supported.