Seaborne Airlines is a small carrier that operates flights in the Caribbean. I just booked a flight on their web site. I found that they offer six different fare classes, but their site has no explanation of what the fare classes mean. I had to call them to ask. It was an easy call to make, and I waited on hold for less than a minute. Nevertheless, I thought I'd post the answer here.

In the order in which they appear on the screen, from right to left:

<b>Economy unrestricted:</b>
You can cancel your ticket for a full refund, or change your flight with no fee. Your first checked bag is free.

<b>Economy restricted:</b>
Tickets are nonrefundable. If you cancel your flight, you will receive an airline credit for the price that you paid, minus a $25 fee. I think the fee goes up to $50 if you don't rebook within a certain period. As with economy unrestricted, your first checked bag is free.

<b>Economy Advance Purchase:
Economy Class Promotional:
Budget Instant Purchase:
Great Sale Fare:</b>
All of these fare classes have exactly the same terms as one another, and the only difference between them is pricing. They always offer Economy Advance Purchase tickets. A certain number of seats on each flight are offered at the Budget Instant Purchase price, which is lower. The other two fare classes are occasionally available as special low-price offerings. For all of these fare classes, the cancellation/change policies are exactly the same as they are for Economy Restricted. The only difference between these classes and the Economy Restricted class is that, with these cheaper tickets, you don't get your first checked bag for free.