I am not sure what the correct name of this part of the car is but I do know that it can get filthy in the rain. Fortunately for me, it does not rain here too much so this is not a big problem. But when it rains a lot, a lot of water and debris can accumulate on the section of the car you step over to get into the seat. I didn’t really notice this too much until I wondered why my pants had unexpected amounts of dirt on them. The service department told me that there is not a fix for this, and engineering is busy working on the model S. They did say that a lot of other Tesla customers have complained about this problem.