Some types of investments are never subject to ad hoc withdrawals

Types of Investments that are Not Subject to Ad Hoc Withdrawals
Withdrawals are never made automatically from money purchases and drawdown pensions, trusts, Venture Capital Trusts (VCTs), and Enterprise Investment Schemes (EISs) in order to meet shortfalls. A withdrawal strategy or planned withdrawal must be set to make withdrawals from these account types.

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Liquid assets are normally used only when needed to supplement income in the cash flow

One exception to this rule are child trust funds, which may be used as the source of ad hoc withdrawals but only for expenses owned by the child (e.g. education expenses), as set on the People panel of the Expenses screen.