Field help (COAPFX) 

Mention the account root (alphabetical, numerical or alphanumerical) attached to the chart of accounts for which it is requested to define a default class.

SEEINFO The numeric prefix may contain up to ten characters. The longest prefixes are taken into account first.
For instance, for prefixes 4, 44 and 445, prefix 445 is used to define the class by default (for a VAT account 445710).