This extra thin printed circuit means the thickness of printed circuit board is more thin than normal PCB. For most of FR4 PCB, Min thickness is 0.3mm or 0.4mm, whatever it is 1L or 2L. For 4L, it will be more about 0.6mm.
 

But sometimes, limited to space or margin, or needed by design purpose, people needs extra thin such as 0.25mm, 0.20mm, or even 0.15mm board, so that PCB can be used in such as SIM card, sensor card, and so on. Extra thin FR4 PCB can be bendable similar like a flexible circuit, but more rigid, and cost will be lower, comparing with the same copper traces.
 

For extra thin PCB, we can make it at 0.15mm, 0.20mm, 0.25mm, 0.30mm, with or without the soldermask, and copper thickness 1/2OZ, 1OZ.



Extra Thin Board Capability
Base material: FR4
Board Thickness: 0.1mm~0.3mm(0.004"~0.012")
Copper thickness: 0.5 OZ, 1.0 OZ
Outline: Routing, punching, V-Cut
Soldermask: Bare/White/Black/Blue/Green/Red Oil
Legend/Silkscreen Color: Black/White
Surface finishing: Immersion Gold, HASL, OSP
Max Panel size: 600*500mm(23.62"*19.68")
Packing: Vacuum
Samples L/T: 7~9 Days
MP L/T: 8~10 Days

If you need board thickness less than 0.10mm, then you should use flexible circuit which can be 0.06mm for 1L FPC or 0.10mm for 2L FPC.