The structure of accounts every entry is written against.
Entries post only to ledger accounts
A group is only a category — assets, liabilities, revenue, expenses. Nothing posts directly to it.
A ledger account is the real one: bank, accounts receivable, sales. Entry lines always land here.
If an account is missing from the picker when writing an entry, it is usually a group rather than a ledger account.
Dimensions are not a third level
There is no third level. Instead there are analytic dimensions: a separate axis attached to the entry line alongside the account, which can link to a real entity such as a customer.
So you define "accounts receivable" once, and the balance per customer comes from the dimension — not from creating a ledger account per customer.
Some accounts require a dimension
A ledger account can require that no line is accepted without a dimension. Receivables and payables are set up that way already.
When an entry is refused for that reason the amount is not the problem — set the line's dimension.
Account codes are ordered
Codes are banded and the order means something; reports group by it. A new bank account takes its code automatically from the banking band.