The optional parameter digits defines the precision and scale of the number.

The precision is the total number of significant digits in the number (before and after the decimal point), the scale is the number of digits after the decimal point.

If the parameter digits is not present, the number will be a double precision floating point number.

Warning: these floating-point numbers are inexact (not any value can be converted to its binary representation) and this can lead to rounding errors. You should always use the digits parameter for monetary amounts.

Example:

'rate': fields.float(
    'Relative Change rate',
    digits=(12,6) [,
    Optional Parameters]),