The numbers that contribute to the significance of the total number value are significant figures. All the numbers are significant digit except all leading 0.
Rules:
Non-zero digits are significant.
If the zeros is in between non-zero digits then it will be significant.
If the zeros are leading then they are not significant.
if the zeros are at the right of the last non zero digit then they are significant (Trailing Zero).
Now consider the number .00000004
Since all the zeros are leading that means they are not significant.
The number 4 is a significant digit as it is a non zero number.
