A geodetic datum describes the origin, orientation and scale of a station network, typically with respect to a reference frame. In the analysis process of Very Long Baseline Interferometry (VLBI) observations, the introduction of a geodetic datum is inevitable for the determination of precise reference frames and Earth orientation parameters (EOP). In general, several methods of datum definition exist within the VLBI community, including Helmert rendering and the no-net-translation/no-net-rotation (NNT/NNR) approach. While the first introduces conditions with quasi-infinite weight, the NNT/NNR method can be controlled by the selection of formal errors. Evaluations of the CONT17 legacy-1 campaign and a longer time series of IVS 24-hour sessions show that the variance information (formal errors) of the estimated terrestrial reference frames based on the different methods can differ in the mm to almost cm range. Neglecting this issue could lead to potential issues when combining or comparing solutions from different analysis centers.