Effective October 16, a number of changes to IATA fare construction rules have come into effect. Some of these rules are fairly esoteric and won't come into play very often; the first one though is useful to the average FTer. At the very least, it's a peek at how crazy these rules used to be. Before reading this I didn't know some of these rules existed, so please bear with me if my explanations are a bit off.

I got this information off of a presentation from Sabre, one of the global distribution systems.

Europe considered one country for open jaw
In order to combine two fares on a half-round-trip basis to create an open jaw, the "open" ends need to be in the same country. Europe is now considered one country for this purpose. (I don't know for sure which countries constitute Europe in this case.)

Example: LHR-JNB-FRA used to price out as two one-way fares. Now it can price out as 1/2 of the LHR-JNB round-trip plus 1/2 of the JNB-FRA round trip. (Of course, all other combinability rules in the fare still apply.)


Constructed minimum fare checks abolished
If there is no filed fare between two cities, a fare must be "constructed" between the departure/arrival city and the connection point (basically, the two fares are added together.) All of the weird checks like HIP checks won't apply within the construction points.

Example: SIN-MNL-NRT-ULN, stopover in each city. There is no constructed fare between MNL and ULN. Previously, IATA required that MNL-NRT be HIP-checked; that requirement no longer exists.
Note that there does exist a SIN-ULN fare, and so the SIN-NRT HIP check does in fact happen.

Increase in arrival/departure limit in Europe
Previously, there was a limit of three arrivals and three departures within one country on a single ticket. (I believe, but am not sure, that this was effective in Europe only, and the rest of the world had a limit of four arrivals/departures.) This limit has been increased to four (I think to be in line with the rest of the world.)

Example: MIA-LHR-FRA-LHR-FRA-LHR-FRA-LHR-FRA-DFW-MIA has four arrivals and four departures each in England and Germany. Before this itinerary needed to be issued on two separate tickets. Now it can be issued on one.

Some minimum fare checks eliminated
The following fare checks are no longer used:

• Directional minimum check (DMC):
  Similar to a HIP check, but in reverse: When stopping/connecting in a city, ensures that the fares of origin-connection and connection-destination is not less than the direct fare. Previously only applicable to one-way and normal open jaws involving Japan and issued outside country of origin.
  
• Country of payment (COP):
  When an international round trip or circle trip is sold in the destination country, ensures that the fare is not less than the same trip in the opposite direction. Note that this check still applies for itineraries originating in Algeria or Tunisia, to or via France/Italy to Europe, when issued in France or Italy.
  
• Common point minimum (CPM):
  When a point is used twice in an round-trip + open-jaw itinerary, ensures that the open-jaw portion is not less than the round trip fare.
• One way sub-journey check (OSC)
  On a one-way ticket, ensures that the fares of stopover-connection and connection-destination is not less than the direct fare. I think the difference between this and the DMC is that the stopover is an intermediate point, not the origin.
  
• Return sub-journey check (RSC)
  Same as OSC, but for round-trip, circle-trip, and open-jaw itineraries.

Note that higher intermediate point (HIP) check is still in place.

Example:
DMC: BKK-NRT-FRA (sold and ticketed in Germany) no longer checks for a higher BKK-FRA fare; if BKK-NRT + NRT-FRA is cheaper that fare is used.
COP: CKG-x/HKG-NRT-CKG (sold and ticketed in Japan) no longer checks for a higher NRT-CKG round trip fare; if CKG-NRT is cheaper that fare is used.
CPM: BOM-NRT-BOM-MAA no longer checks to see if NRT-BOM + BOM-MAA is higher than NRT-MAA. This would not normally be a problem if the BOM-NRT segment not part of the itinerary (note that if NRT-BOM is higher than NRT-MAA and BOM was a stopover, you'd get a HIP check).
OSC: SGN-PNH-xBKK-DAC no longer checks to see if PNH-DAC is higher than PNH-BKK + BKK-DAC.
RSC: KIX-PUS//ICN-PEK-ICN-KIX.