If you just require 70 µm without specifying more, the manufacturer will likely ask what it is you really want. The good ones at least will ask.
I typically specify the starting copper thickness, and then require the final thickness to follow IPC class 2. On my PCB specifications I write clearly that the number is the starting thickness.
The standard starting copper thickness are 12, 18, 35 and 70. On inner layers the final thickness is smaller due to cleaning processes and on outer layers it is thicker due to plating.
Minimum final thickness for IPC class 2 are