The last time I left Thailand for Laos (late March) at the Chiangkhong to Houayxai crossing I made sure that I could leave at the Mueang Ngeun/Nam Ngeun to Chaloemprakiat (Nan) crossing. This crossing is also known as the R2C40 crossing on the green temporary import document at the Departure Date/Crossing as shown below.
As the lady at the Laos border crossing did not know what the Mueang Ngeun crossing was, I gave her one of my maps which showed all the border crossings so she would know the names of the border crossings in English. She had a incomplete list of border crossing names in Lao language and she was so kind to make a picture for me showing the names and the codes they use for the exit points.
Jean Baptiste was so kind to help me with the translation of the names of the border crossings with its codes:
Boten Crossing - R1C10
Panghok (Sophoun) - Tay Trang - R1C40
Nam Soy (Nam Xoy) - Nameo - R2C20
Nam Hueang (Nakrasaeng) - Thali (Loei) - RC30
Nam Ngeun - Houay Khon (Nan) - R2C40
Nam Khan - Nam Can - R3C50
Friendship Bridge 1 Vientiane - Nongkhai - R5C10
Friendship Bridge 2 Savannakhet - Mukdahan - R3C10
Friendship Bridge 3 Thakhaek - Nakhon Phanom - R3C30
Paksan (Pakxane) - Bungkhan - R3C40
Nam Phao (Lak 20) - Keoneua/Cao Treo - R3C50
Na Phao - Chalo - R3C60
So next time you want to cross at another border crossing then the one you entered, make sure they use the right code as otherwise you may have problems to leave unless you are at the correct crossing mentioned on the green form. In the worst case the border crossing people will refuse to let you leave and you will have to return to the border crossing mentioned on the green form or, if you are lucky, just pay a fine (600 Baht with receipt at the Vientiane/Nongkhai crossing or 500 baht without a receipt at the nam Ngeun to Chaloemprakiat crossing).
Generally the customs people will insist that, when you enter at one point, they want you to leave at the same point. However, the customs people at the Vientiane to Nongkhai crossing explained that you can leave at another border point but that for administrative reasons you will have to indicate the border point where you want to leave. This is so that the customs people at the entry point can send the copy of the green form to the exit point so both the original and copy can be filed at one place.