These are advanced calibration procedures that ensure that VNA can provide the dynamic range and accuracy that these instruments are known for. These procedures require precision standards at the point of calibration - that is, at the end of the measurement cable, probe or fixture. The abbreviations identify which standards are required for calibration - SOLT for Short-Open-Load-Thru, TRL for Thru-Reflect-Load. The user needs to be very thorough when performing these calibrations to achieve the desired accuracy, which can be difficult for a novice user. Standard calibration kits are available at the cable-end calibration and some are available for probe-based calibration. However, even with custom-designed standards, the fixture level calibration is not always possible.
Whereas a TDR oscilloscope operates perfectly well with a simple vertical and time base calibration, its accuracy can also be increased by utilizing these advanced calibration procedures. However, as for a VNA, the higher accuracy achieved by using these procedures comes at a price of increased difficulty of use of the instrument.