網絡搜索學習了一下，僅供參考。大概相當於公司給員工發股票獎金，一部分賣了預交收入稅，淨額股票存托管，未來賣了差價算資本利得稅。

For example,

Grant date: $40,000 for 4 years vesting. Generally no tax implications.

Vesting date of the 1st year : Vested stock value $10,000 = $25 per share x 400  shares - $25x 100 shares ( to withhold and pay taxes ) = Net $25 x 300 shares = $7,500 deposited in Fidelity or ETrade and become cost basis. $10,000 is  shown in W-2 as ordinary income and $2,500 as withholding taxes.

Selling date: ($60 selling price - $25 cost ) x 300 shares = Capital Gain shown on form 1099-B