If we don't take a final sector?
 

I purchased RTW from Seoul startiing ICN-HKG-DXB .......
The last sector will be NRT-ICN to complete the RTW.
If I do not take this last flight and finish the RTW in JAPAN, is there any penalty not to complete RTW?

Also, I might not take one flight at the middle of RTW and use ground transportation, does it require re-issuing the ticket because of re-routing?

Thanks in advance.

Technically, there could be. Practically, no. I would recommend just changing the final sector date to a few days before the ticket is set to expire, then just let it lapse. Note that doing so will require that NRT be a stopover, of which you're only allowed 2 in the continent of origin.

Yes.

Unless it's a paper ticket with open dated coupons, in which case just cancel the sector reservation and throw the coupon away.

Thank you, both of you!

<<Note that doing so will require that NRT be a stopover, of which you're only allowed 2 in the continent of origin.>>

Actually, our flights are:

ICN-HKG(transit)-CKG, then take another airline to go Lhasa and Lijiang, back to CKG-HKG(another transit)-DXB-AMM-LHR(transit)-LIS-LHR(transit)-LAX(Sidetrip to Honolulu by another airline where we live , and then back to LAX to continue the RTW after 6 months) LAX-EZE-LIM-LAX(transit)-NRT(stopover)-Haneda-Gimpo.
The last sector(Haneda-Gimpo) will be the last day of 1 year limit.

I have never heard of any airline imposing a penalty for missing part of any ticket, but contractually they can collect an additional charge. For OWE it would be computed as point-to-point fares for the sectors flown, minus the fare paid. This could total well over USD 10,000 depending on your routing. As the industry struggles with tough times, that huge penalty must start to look very tempting to some accountants ....

Sometimes, discounted round trip fare is cheaper than normal one way fare.
If we purchase the round trip, and then do not show up for the return flight, airlines have right to charge the difference of those two fares, but usually they do not enforce their right to do so.
In the case of RTW ticket, who checks missing flight, who calculates the fare difference(maybe issuing airline) and how they allocate the charge collected.

It sounds very complicated, but we still have risk to be charged:(.

I know the answer to the first 2 questions: the airline selling you the ticket determines the addcol, if any. So they control both the conditions triggering the addcol and the computation of the amount of addcol. I believe the allocation is to the ticketing airline (they don't have to split the money with the rest of Oneworld). The legal ramifications are fairly deep, so I think AA would not do this except in cases of abuse (a pattern of multiple tickets where some legs are not flown and a deliberate circumvention of the fare rules). Fwiw, AA has successfully sued some pax for throw-away ticketing (buy round-trip and throw away the return coupon). With e-tickets this is much more visible than it used to be.