The U.S. Senate narrowly approved a tax overhaul in 2017 that included a proposal designed to generate more tax revenues for the federal government by restricting choices for some taxable investors. The proposed change is all about timing and two acronyms—HIFO and FIFO. HIFO stands for highest in, first out and FIFO stands for first in, first out.

Under tax law at the time, investors with taxable accounts were free to choose which shares of a specific company they would like to sell. Under the proposal, investors must sell their oldest shares first. While that doesn’t sound like much of a change, it’s a big deal from a capital gains perspective.

Here’s a simple example: You purchased 100 shares of Flooble for $10,000 in 2000. At the time, the shares cost $100 each. In 2005, you purchased 50 shares of Flooble for $10,000. Those shares cost $200 each. In 2015, you purchased an additional 25 shares, paying $10,000, or $400 a share. Each time you purchased a block of shares you created separate tax lots. In this example, you would own three tax lots. Currently, Flooble sells for $300 per share. You have a total of 175 shares, which are worth $52,500, so you’re ahead $22,500.

Or are you? Under current law — for tax purposes — you could show a loss. You could choose to sell the 25 shares you purchased last using HIFO accounting. Remember, those shares cost you $400 each when you purchased them, but they are now worth just $300 each, so technically you lost $100 per share. Choosing to use HIFO shares means you could use this loss to help offset other gains, which works to minimize or eliminate capital gains taxes within a taxable investment portfolio.

The Senate’s 2017 proposal would have eliminated your flexibility. If you wanted to sell shares of Flooble, you would have been forced to use FIFO accounting. In other words, you would be forced to sell some of the shares from your first tax lot. In this example, it would be the shares you purchased for $100 each. If you sold 25 of those shares, you would have a taxable gain of $200 per share, since you purchased the shares for $200 less than what they’re currently selling for ($300 per share).

In either case, you now own 150 shares of Flooble, but your tax position is vastly different. Under current law (using HIFO), you could report a loss of $2,500 from the transaction. Under the proposed law (using FIFO), you would have been forced to report a profit of $5,000, which would be subject to capital gains taxes.

The FIFO provision in the Senate’s tax proposal created significant tax concerns for investors with taxable portfolios. (The Senate’s bill excluded mutual funds from this provision.) To their credit, lawmakers moved swiftly to remove the FIFO requirement from the proposal.