1.0mm is the Tracer Probe.
The milling cutter has 3 optional size: 1.5, 2.0 and 2.5.
No difference except size (length)
You can use 1.5, 2.5 on everything.
The 2.5mm cutter will do almost everything. 1.5mm cutter will be used on things like hu162 for new vag and I believe internal ford hu101.