What does it mean that Siemens vl 160x circuit breaker is marked tm?

Tm refers to thermomagnetic release.

Thermomagnetic trip:

It has two functions: thermal trip and electromagnetic trip. Thermal trip is to push the trip transmission mechanism through the over-current delayed heating deformation of bimetal; Magnetic trip is to push the armature to drive the trip through the short-circuit current of the electromagnetic coil instantly.