Additionally, many macromolecular motions have a highly anharmonic character (e.g. rotamer jumps or sub-domain opening) that manifests in discrete but weaker density around distinct positions with no continuous density connecting the states. This type of heterogeneity is not well fit by B-factors, which leads to underestimating the displacements present in the experimental ensemble . To overcome this limitation in the PDB format, atoms can be replicated and labeled with an "alternative location indicator" (altloc), signifying discrete states. Refinement and validation programs treat atoms sharing the same altloc as having the ability to interact with each other and with atoms lacking an altloc, but not with atoms with different altlocs. However, the lack of a hierarchical relationship between altlocs restricts the complexity of information encoded by the legacy format.