What is the maximum PIC size for a COMP-3 field?

Problem:

Defining a field with  S9(31) COMP-3 returns the message :

* 213-S*****************************                                         **

**    Item is longer than USAGE allows or contains too many numeric positions

Resolution:

According to the ANSI standard, numbers are limited to 18 significant decimal digits.  All significant digits must be within 18 digits of the decimal point.