Sk also has negotiated fares that you cannot look up in normal GDS. These can be spotted if you have a ticket where it says "IT" under FARE. SK however adds the line 'for conditions refer to GG SK AIR GERMANY' in the endorsement box. I've stood at a LH counter for ages while a LH ticket agent read thru all of the text under GG SK xxxx to find out if the fare allowed a stopover in ARN.

If you have a negotiated IT fare, SK/LH ticket agents don't want to touch your C/J tickets with a 10 foot pole. I get my TA to add "changes allowed" to the usual 'SK only/No rerouting' text in the box. That helps quite a bit.