Short version: if you own a large block of shares, the selling of those shares may change the price of the shares. So how do you value them for accounting purposes?
CFO.com reports that FASB is considering allowing more judgement when it comes to the valuation of these blocks. Currently this is only for brokers and dealers, but the same logic can be applied to nearly all large block holders.
Securities Blocks and Fair Value - - CFO.com
""This has been a contentious issue for the board for a long time," says board member Michael Crooch. "If you had a large, 10-million-share block of General Electric, you may have to sell it at a discount [relative to price-times-quantity], or you may actually have to pay a premium for a large block like that if there was heavy demand.""