Cash and bank — business logic

A cash document records money movement: payment to a supplier, receipt
from a customer, transfer between the company's own accounts, payment for
services. It doesn't move goods — only money, and lives by its own separate
logic, not tied to warehouse lots.

Manual and automatic

A cash document can be created by the user manually (on its own), or
automatically — as a "twin" of a receiving or expenditure/selling document,
if a payment account is specified in their header. In that case payment is
processed together with the goods document, same number and date, with no
separate step.

Why this matters: without this link, every payment would require two
separate documents and two separate user actions, and it's easy to forget
to process the payment separately — goods shipped, but the money side of
the operation left unrecorded.

A line is a transaction on its own

Every cash document line is a complete transaction in itself:
- Debit / Credit — money leaves or arrives. Usually only one of the two
amounts is filled; both at once is a rare case of a compound entry (e.g.
a currency conversion with a fee).
- Destination account — for a transfer between the company's own
accounts: one account "from," the other "to."
- Linked document — which receiving/expenditure/sale is being paid by
this line.

What happens on processing

For each line: if the link to a related document isn't set yet, the
current document's number and date are used; if a counterparty is
specified, their balance is updated (the difference between debit and
credit is added to the balance: paying a supplier reduces the debt owed to
them, receiving from a customer increases what they owe, or reduces what's
owed to them). A cash document by itself never recalculates stock — it's a
purely monetary operation, fully separate from lot accounting.

Services aren't a separate document type

Paying for a service (a consultation, delivery billed as its own line item,
etc.) is entered as an ordinary cash document line — its meaning is set not
by a document type, but by the line's classifier ("operation reason"), the
same as any other transaction. There's deliberately no separate "service
document" form with special rules — that would be an extra type just for a
special case that's perfectly well described by the general cash document
mechanism.