Unusual NT for
Lower 2 Unbid
An Unusual NT is normally a 2NT overcall of a 1-level opening bid. If the opening bid is a major, the Unusual NT promises at least 5-5 in the 2 minor suits.
If the opening bid is a minor, you can either agree to play the Unusual NT to show the 2 lower unbid suits or the 2 minors. Since most people play 5-card majors, they often have to open the bidding 1 or even 1 with a 3-card suit. Therefore, it is very possible for overcaller to have 5 of the opening minor suit.
Then if you do have a 2-suited hands with Hearts and a minor, you can start by overcalling Hearts and bid the minor later if you get the chance.
However, the majority of people play Unusual Notrump for the lower 2 unbid suits.
After the opponents have both bid, it is possible to play 1N as Unusual. If you are a passed hand, the assumption is that it is Unusual. If you are not a passed hand, you must alert the bid as being Unusual.
When 1N would be unusual showing 5-5, then 2N can be used to show even more distributional hands - at least 6-5. So then you have Double to show 4-4 or 4-5, 1N to show 5-5, and 2N to show 6-5 or more.
When LHO has opened, 2nd chair has passed, and RHO makes a 2-over-1 bid promising 11+ HCPs, you must have a very shapely hand to make any kind of call since the opponents clearly have the majority of HCPs. For example, after 1H-P-2D, a double should show 5-5 or more in the unbid suits, per Mike Lawrence in his book, Takeout Doubles.
Since Double shows 5-5 or more, you don't need to use 2N for the unbid suits. This also saves you space after something like 1D-P-2C, since if you double, partner can respond on the 2 level versus the 3 level if you bid 2N.