Pursuant to 840 CMR 23.01, a realized gain (loss) is any profit (loss) sustained on the sale or maturity of any investment of any system, due to the amount received being more (less) than the book value on the date of its sale or maturity. An unrealized gain (loss) is any amount by which the market value of any investment required to be valued at its market value pursuant to M.G.L. c. 32, § 21, paragraph (b) is more (less) than the value at which such investment was included in the assets of the system on the date of the last previous valuation.
840 CMR, § 23.02