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Your open jaw #2 describes MSP-SFO-BOS. Personally I don't think that is an open jaw (although I don't claim to be an open jaw expert in scenario number 2)
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This is the general idea behind an open jaw:
" Open-Jaw Trip means travel that is essentially of a round-trip nature except that the outward point of arrival and the inward point of departure are not the same or the outward point of departure and the inward point of arrival are not the same. Open jaws are allowed (unless there is a restriction in the fare rules) when the mileage between the open-jaw points is equal to or less than the mileage of the shortest sector flown. "
So, unless I'm wrong, MSP-SFO-BOS could be considered as an open jaw because the distance between MSP and BOS (1124 miles) is less than the distance between MSP and SFO (1589 miles) an the distance between SFO and BOS (2704 miles).
I use open jaw tickets type # quite often (I had an ORD-CDG-SEA flight last month).
Less common are either double open jaws or open jaws between two countries (I've booked a MIA-SYD-YUL ticket two weeks ago on Air Canada).
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What if you went SFO-MSP-BOS, that's still scenario number 2. I can't see how that could be an open jaw, that's a one way trip with a stop, or again two one ways.
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You are right, this is not an open-jaw becase BOS-SFO is a greater distance than the flown segments.
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If I leave LAX, go to ORD and return to SFO is that an open jaw?
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Yes
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If it is, then same scenario and you "return" to DFW is it?
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No but DFW-LAX-ORD could be
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... Or said another way you can't have an open jaw on the departure/return city only somewhere in-between.
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You can even have an open jaw on both the inbound and the outbound end, it is called a double open jaw.
I hope this helps,
Regards,
FRAC
BTW, the open jaw definition is from
Alaska Airlines. I am sure there is a formal 'IATA' definition but I don't have access to it.