Rules for payment of securities transactions executed in accounts are established under Federal Reserve Board Regulation T. Under these guidelines, purchases in cash accounts can be accepted under the following conditions: if there are sufficient funds in the account to fully pay for the purchase  at the time the trade is executed or the customer makes a good faith agreement to promptly make full payment for the purchase on or before the settlement date and before selling the security.
Settlement date may vary by security type and conditions of the trade but is generally two business days for equities and one business day for options and most mutual funds. Fixed income security settlement will vary based on security type and new issue versus secondary market trading.
It is important to note that the definition of sufficient funds in a cash account does not include cash account proceeds from the sale of a security that has not settled. It also does not include non-core account money market positions.