iCalendar is a computer file format which allows Internet users to send meeting requests and tasks to other Internet users, via email, or sharing files with an extension of .ics. iCalendar is used and supported by a large number of products, including Google Calendar, Apple Calendar (formerly iCal), IBM Lotus Notes, Yahoo! Calendar, Evolution (software), eM Client, Lightning extension for Mozilla Thunderbird and SeaMonkey, and partially by Microsoft Outlook and Novell GroupWise.
VSys One can create and include iCalendar files when it sends e-mails to people based on scheduled events, such as job assignments, trainings and interviews. Whether a letter includes .ics files depends on the letter's content and on if you've checked Include iCalendar/ics files with e-mails in the letter's properties.
Different e-mail programs on the volunteer's end will handle iCalendar files differently (if at all).
Sample iCalendar file
		BEGIN:VCALENDAR		
		VERSION:2.0		
		METHOD:REQUEST		
		PRODID:-//Bespoke Software///VSys One//EN		
		BEGIN:VEVENT		
		UID:JBUEOVNLUDRO7D7H		
		DTSTAMP:20140328T062801		
		DTSTART:20120826T090000		
		DTEND:20120826T093000		
		CATEGORIES:Interview		
		ORGANIZER;CN="William A. Cornett":MAILTO:xxx@vsysone.com		
		LOCATION:Main bldg. floor 2		
		ATTENDEE;CUTYPE=INDIVIDUAL;CN="Veronica Bergin":MAILTO:x@x.x		
		SUMMARY:Follow-up - Veronica Bergin		
		DESCRIPTION:Follow-up - Veronica Bergin\n		
		 123 Main Street\n		
		 Schenectady, NY 12345\n		
		 USA		
		END:VEVENT		
		END:VCALENDAR