I am not saying anything very deep. We can try to count all the different ways 5 (or 100) people can have sessions on a website
#1 |--------------------|
#2 |---------------| |---------|
#3 |------------| |---|
#4 |----| |------| |--|
#5 |---------------------|
Each of these channels have overlapping event. Some sessions happen strictly before the other. Sometimes, one logs on before the other one logs of. Etc. The more channels we have the more complex behavior. That is all the paper is saying.Sorry about that.