Paul, sorry long answer needed
Both Ferrari cars were underweight by 1kg and both cars had under thickness planks by 0.5mm
Plank is basically a fibreglass/phenolic resin laminated slab that is machined flat, bolted to the chassis and closest part of the car to the ground
Does 2x things, prevents the bottom of the floor/chassis rubbing away and limits the teams running the car too low to squeeze out more aero performance
Planks have flush titanium skid blocks which are limited in surface area and strategically positioned to protect legality holes where the FIA measure wear ie minimum plank thickness
Can't remember the exact rules but the plank is a specified size eg 300mm wide, front edge and rear edge specified, max. thickness 11mm, min thickness 9mm. Material density between 1300kg/m3 and 1450kg/m3, most teams will run higher density unless they're struggling to hit mimum weight
Beliw FIA tech drawing is out of date but the principle is still the same
Teams won't mind the planks being rubbed to below 9mm min. limit, they'll actually actively try to rub it down thinner scratching for aero performance, as only the thickness around FIA legality holes are measured
If Ferrari are below legal limit around the legality holes they'll for sure rubbed more plank away in other areas
Doesn't take much plank wear to lose a 1kg eg 300mm wide x 3m long area of 1mm wear on a 1400kg/m3 density plank is 1.25kg
All teams monitor plank wear very carefully as the lower the car the more aero performance ie dozens of thickness measurements and even skid thermocouples to measure tempersture and strain guages to measure impact loads
F1 is all about the finest of margins, pushing every FIA legal limit/legality test and performance parameter as close to the limit as possible
Not a case of Ferrari cheating they just found out where the limit is for ride height
Bad job by them, good job by the other teams, McLaren, Mercedes and RBR will be very happy as they know Ferrari will take a slightly more cautious approach to ride height which will cost them aero performance until they get the confidence back on pushing the wear limits hard