America's cryptocurrency tax policy is confusing everyone—and it’s high time for the US Internal Revenue Service to sort it out and explain. That’s the gist of a strongly worded letter sent by a bipartisan group of 21 members of the House of Representatives to the IRS on April 11.

The legislators, led by Tom Emmer from Minnesota, join a growing chorus of policy advocates calling on the IRS to update the guidance it published in 2014. There remains “substantial ambiguity on a number of important questions about the federal taxation of virtual currencies,” they write. Emmer and his colleagues say there’s an “urgent need” for additional guidance.

At the top of the priority list are open questions around the concept of capital gains, which are profits an investor realizes when selling an investment. The guidance the IRS published five years ago stated that “virtual currency” would be treated as property for federal tax purposes. Buying and holding it is not a taxable event. But if you use your crypto to buy anything—even something like a cup of coffee—you have to make sure you keep track of the difference between the value of the cryptocurrency when you first bought it and when you spent it. If it increased in value, that counts as a capital gain, and you have to pay taxes on it.