What do the [GND Clamp] and [POWER Clamp] keywords describe in an input/output buffer information specification (IBIS) model?

Description The [GND Clamp] keyword describes the I (current) and V (voltage) relationship when the input or output pad of a buffer is driven below ground (or the reference specified by the [GND Clamp Reference] keyword).For an I/O buffer, this is the ground-relative data gathered while the buffer is in the high impedance state. Meanwhile, the [POWER Clamp] defines an I-V relationship when the input or output of a buffer is driven above Vcc (or the reference specified by the [POWER Clamp Reference] keyword).For an I/O buffer, this is the Vcc-relative data gathered while the buffer is in the high impedance state.