The value is assembled with the first 4 chars of the company name followed to what seems like a random number of 5 digits, separated by semicolons (multi value format). The latter one is to be sure that the accountancy reference is not lost we duplicated companies are merged.
I haven't used this field value on any project so far actually... the added value seems low.
It's also hardcoded logic from Efficy, you can't and should not try to customize it. If you need something similar, hide this standard
ACCOUNT_REF field by placing
SYS_FIELDS and by making a new custom field with custom logic, preferably calculated in the
BeforeCommitComp workflow event.