It's about icing layers
630004
The first digit is always a 6.
The second digit it the type of icing...3 = light icing in precip.
The next 3 digits represent the base of the layer in 30's metres (or 100's of feet)... 001 = 30 metres; 100 would mean 3000 metres.
The last digit is the thickness of the icing layer in feet. 4 = 4,000 feet or 1200m.
There are also codes for other types of weather, but the 6 indicates in this case it is icing.