The number of digits that can be written in the fractional part
of a floating point number.
For example if the value was five then the smallest number which
could be represented would be 0.00001.
The PDF 1.4 specification limited this value to five digits. You
might ask how relevant the PDF 1.4 spec is now - after all it came
out in 2003 and then was superseded in 2004.
However the PDF/A spec is designed against the PDF 1.4 spec so
is is not just that the document might display badly in a version
of Acrobat from fifteen years ago.
This value defaults to five and may be set to any value between
five and thirty-eight.