The penetration level of distributed energy resources (DERs) is increasing and has significant impact on the voltage stability of distribution networks. Based on the various types of DERs with distinct reactive power characteristics (RPC), their different contributions to the system voltage stability require classification. Firstly, the features of DERs are reviewed and classified based on their RPC, to investigate different distributed generation technologies for reactive power support in distribution networks. Then, the concept of a relative available transmission capacity index (RATCI), which is based on power transfer margin of the power-voltage curve considering the non-negligible distribution network resistance, is proposed to quantify and evaluate the voltage stability by integrating DERs with the defined reactive power types. Case studies have been conducted for an IEEE 33-bus distribution network to calculate the system RATCI for the mixed integration of DERs. Results show that the multi-type and multi-locational integration of DERs can improve the voltage stability of a distribution network.