For evaluating the capacity of wind powers, photovoltaics and other power electronic grid-connected units supporting power systems, the core foundation is to correctly understand the unit's functional role (i.e., the unit characteristics) that unit adjusts its own internal voltage amplitude/frequency according to the active/reactive power imbalance. However, the mainstream PLL-based grid-connection structure in power electronic units seriously hinders the understanding of the unit's functional role. In particular, based on a specific PLL-based grid-connection structure, the industry and academia at present form a "grid following" role perception that the internal voltage of unit follows the grid voltage or terminal voltage, and have not recognized the functional role that the unit should take during the system operation. Therefore, through an in-depth understanding of the independent excitation-response mechanism of current control which is hidden under the PLL-based grid-connection structure, i.e., the internal voltage response depends on current excitation alone, the functional role of PLL-based grid-connected units in which the active/reactive power imbalance independently adjusts the internal voltage amplitude/frequency is clarified. Afterwards, a role characterization method for unit is proposed based on the relationship between active/reactive power imbalance excitation and internal voltage amplitude/frequency response, i.e., the amplitude-frequency motion equation. Finally, the inevitability of characterizing the role of PLL-based grid-connected units through the relationship between power excitation and internal voltage response is elaborated on, and the existing limitations in the understanding of the role of PLL-based grid-connected units in industry and academia are pointed out.