An event that is a meeting has a user-modifiable property called "Busy Status". You can set this to Free, Busy, Out of Office, or Tentative. This is a user property and has nothing to do with the dashed lines.
A meeting has attendees. Those attendees have a property that defines if the attendee has accepted, rejected, or not-answered. The not-answered value is called "Tentative" in server-speak.
So when you are invited to a meeting - perhaps on Facebook, or via email, or via Google's meeting request, or Exchange -you are automatically "tentative". You cannot *see this value, *cannot edit it, and has nothing to do with the Busy Status.
Now the problem is if its an iOS event, you need to accept/decline this event in Apple Calendar. Apple does not allow us to modify this.
*If this is a Google event, Informant should allow you to accept/decline the event - therefore making it no longer "tentative", but either rejected or accepted.
So in short - Informant shows a dashed line if you are invited as an attendee to a meeting but you have not rejected/declined the meeting.
Also note, there is an option "Show Tentative Meeting Status" under View Settings->Calendar View->Interface that is on by default and can be turned off - thereby turning off the dashed lines.