The Challenge is a way to more quickly qualify for AAdvantage Gold or Platinum (even Executive Platinum according to rumors?). To achieve this, you contact AA via telephone, pay a non-refundable registration fee, then comply with the parameters of the challenge. Typically the parameters include a certain amount of flying in a 90-day window of your choice. The system uses points, not miles, to measure your progress, so higher-fare tickets get you qualified more quickly. There is a lot of very specific information about it at the AA FT forum.

The only thing I found on Iberia.com was a cryptic reference to an 8kg limit for flights governed by the German Aviation Authority (presumably meaning those departing from German airports). Otherwise, all references to limits only refer to size, not weight.