As renewable energy sources progressively replace traditional synchronous machines

power system strength continues to decline

increasing the risk of system instability. Grid-Forming(GFM) devices can enhance system strength

but their support capabilities lack a clear theoretical foundation. To address this issue

this paper proposes a system strength-based evaluation framework to qualify the GFM and support ability of device. First

system strength is characterized using the norm of the sensitivity matrix of voltage and phase to power disturbances

assessing small-signal stability and robustness requirements. Second

device support capability is defined as the ability to reduce voltage and phase sensitivity

and a method based on the partial derivative of singular values is proposed to quantitatively evaluate devices' voltage and phase support capabilities across different frequency bands. Finally

we recommend classifying devices with both voltage and phase support capabilities as grid-forming devices and those with single-dimensional support as supporting devices. This study provides a theoretical foundation for designing and deploying grid-forming and supporting devices to build safe and stable renewable-rich power systems.