Many location problems assume that, once facilities have been located, customers will be allocated to the closest open facility. However, this is not guaranteed by the model and, as a consequence, must be explicitly imposed by adding some closest assignment constraints. Moreover, the concept of "closest" is easily extended to the one of "most preferred". In this paper, several classical closest assignment constraints appeared in the literature are analyzed. Then, a model which considers location with most preferred allocation is stated: the Simple Plant Location problem with Order. Finally, some valid inequalities which strengthen this formulation are shown.