This phrase would work in a broader context (not specific to an attorney's letter of advice)
There are multiple definitions - click on the translation that matches your context.
(BUSINESS) a commercial letter informing someone of something eg that a consignment of goods has been dispatched (=sent)
Always include your own reference number in a letter of advice, which will usually be the file number allocated to the matter by your company or department.
a letter written by a lawyer to a client advising the client on some aspect of the law
It is common for a letter of advice to be initially sent to the client as an email attachment.