Wow, I already got an answer to the first part of my question. I thought I let you know:

The HIP rule is triggered if you do a stopover.

A backhaul calculation is necessary in a SITI journey when the fare from the point of origin to an internal stopover point is higher than the fare from the origin to the destination resulting in the fare being raised to the higher intermediate fare or HIP.

A backhaul calculation is necessary in a SOTO journey when the fare from the point of origin to an internal stopover or connection point is higher than the fare from the origin to the destination resulting in the fare being raised to the higher intermediate fare or HIP.

The backhaul minimum (plus-up charge) is calculated by subtracting the origin-destination fare from the origin-HIP. The total fare is calculated by adding the backhaul/plus-up charge to the HIP fare.
(as far as I understand this is quoted from some worldspan training material)


Remains the question regarding the mentioned side trip ...L-M-L-...

Any idea? All I know is that you can go through intermediate points more than once as long as that isn't the origin or the destination of the trip. (But if you are able to travel through intermediate points more than once, every loop could be condidered a side trip triggering the HIP rule - or am I wrong?)