I found out that the values are from domain /sapsrm/bbp_acc_dist_ind. Its value range has three fixed values P, Q and V.
I don't know at which point the dropdown box is initalized during runtime.Is it even possible to remove a fixed value of a domain during runtime? I don't think so.
Another approach would be an error message saying "cost distribution by value not allowed". But that solution would be suboptimal.
regards
José