Fiduciary Accounting Income is defined in IRC Section 643(b):

(b) Income

For purposes of this subpart and subparts B, C, and D, the term “income”, when not preceded by the words “taxable”, “distributable net”, “undistributed net”, or “gross”, means the amount of income of the estate or trust for the taxable year determined under the terms of the governing instrument and applicable local law. Items of gross income constituting extraordinary dividends or taxable stock dividends which the fiduciary, acting in good faith, determines to be allocable to corpus under the terms of the governing instrument and applicable local law shall not be considered income.

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