A common configuration for digital pattern instruments is to connect the output terminals of the instrument directly to the DUT. If the digital pattern instrument is connected to a high input impedance DUT, the configuration is referred to as high-impedance load termination.
When using high-impedance load termination during DC operation, minimal voltage drop occurs across the source impedance of the digital pattern instrument due to low current flow in the high-impedance signal path. When using high-impedance load termination during AC operation, the signal reaches maximal amplitude at the DUT.
Digital edges reflect off the high input impedance DUT and propagate back towards the digital pattern instrument. If you match the transmission line to the 50 Ω input impedance of the instrument, the reflections will be absorbed at the instrument and will not impact signal integrity. If you poorly match the transmission line to the 50 Ω input impedance of the instrument, additional reflections will be generated in the transmission line and will negatively impact signal integrity.