There are multiple answers to this depending on the application. The first relates to protein ID. If you have sequenced (via tandem MS) more than one peptide from the same protein, that is multiple assignments. The second relates to charge state at the MS-level. If you have multiple charge states the peaks corresponding to more than one charge state constitue multiple assignments (i.e., a series of charge state peaks). The third relates to the isotopic series. You can use the heights of the isotopic pattern to create a vector that can be compared to the predicted isotopic vector of an elemental composition using isotopic vector angle analysis.