Phasor measurement units (PMUs) have been widely and successfully used in power transmission grids. PMU technologies could be exploited in distribution grids to manage the impact of distributed generation and the electrification of the transportation sector. However, distribution grids are quite different from transmission ones resulting in more stringent accuracy requirements for the distribution PMUs (D-PMUs)). Currently, no standards for D-PMUs are available. However, there are numerous requirements proposed for D-PMUs that need further understanding and assessment. In this paper, a comparison between the IEEE Std C37.118 that is applicable to T-PMUs and requirements for D-PMUs is documented. A classification based on accuracy is proposed. Finally, the factors influencing the accuracy of D-PMUs are discussed in detail.