Pre May 1997, you used to be able to postpone gain from the sale of your main home provided you bought a new home that cost as much or more than the price you got for your old home.
That law was repealed and replaced by the new one (exemption if you owned and used your home as your principal residence for 2 out of the last 5 years).
If you have postponed gain, you enter it on that line.
