What is the difference between a Multimeter (DMM) and a "Two Pole Voltage Tester" (TPT)?

These two products families have different applications and follow different safety standards

Multimeter (DMM) and a Voltage Tester (TPT) are electric measurement tools which are referring to different safety standards.

A digital multimeter (DMM) is a "multi measurement instrument" to provide voltage, current, resistance and also other measurements, and has usually a high impedance at voltage measurement (10 MΩ).

Usually a DMM is battery driven and will NOT display voltage when the battery is dead or a different function is selected (e.g. resistance, continuity, capacitance, current).
Even there is a high risk that a DMM could be set to current measurement and will create a short circuit followed by an arc flash!
Also multimeters and the used measurement accessories are built acc. to IEC/EN 61010 and require a limitation of the probe tip length <4 mm for CAT III or CAT IV rating.

A Two Pole Tester is a tool to test the "ABSENCE OF LIVE VOLTAGE" (acc. to EN 50110), and has typically an impedance below 1 MΩ, most products even below 100 kΩ.

The most basic requirement of a TPT is the indication of voltage above ELV limit (50 V) without any energy source, that means even with dead batteries or without batteries.
This indication must be provided in every function, range or position of switches. Range switches are not allowed. 

A Two Pole Tester (TPT) does not require to have a digital display, most of these products provide only an analogue bar graph.
Even if there is a digital display a TPT may not provide the same resolution and accuracy as a DMM.
The main task of a TPT is to show the absence of voltage in compliance to the 5 safety rules for safe working according to EN 50110!

A load test for TPT is optional and not a basic requirement according to IEC/EN 61243-3.
Also the two test buttons are not a basic requirement and only required if the voltage tester has a test current above 3.5 mA.
The function load test may be required if the TPT has a high impedance above 100 kΩ, for a clear differentiation of operating voltage against interference voltage.


What about the requirements for the test probe tip length?
Why are two pole testers CAT IV rated and have a probe tip length longer than 4 mm?

A voltage tester is built according to the product standard IEC/EN 61243-3 (not IEC/EN 61010) which allows a test probe length of max. 19 mm.
This means that a voltage tester can still be specified (according to IEC/EN 61243-3) as CAT III or CAT IV, even if the metal probe part is > 4mm.
The background here is a reliable connection within socket outlets, terminal blocks or connectors for the safe display of the absence of voltage.

The plastic caps enclosed with the product are not required for adaption to CAT IV rating.
These plastic caps are only required in the UK according to the national regulation GS38.