Question about correspondence references

[macgrl]

Hello all As a slight follow on to my last thread. I have a question about client and correspondence referencing. I regularly have letters from lawyers, accountants and other business, all of which have a "your ref" section on the letter with a code for how they have referenced me as a client. This is something I want to adopt too and would like to know of any rules , conventions or systems for doing this. I have tried to work out some logic for how some business reference me but can't Any help greatly received