September letters are hard to get. They do exist but conditions have to be right. The conditions are as follows and they are based on US immigration law:
1) Your conviction should not be a crime of moral turpitude. There are few excludeable offences which are not considered involving moral turpitude. The most common are:
* DUI
* Common Assault
* Dangerous driving
We can get an idea of what is considered a crime of moral turpitude just by browsing the official US immigration website.
2) Your said crime or crimes were committed when under 18 years of age and if incarcerated, you were paroled no more than 5 years later after becoming 18 years of age. This means when the crime was committed under 18 and you were released from jail before 23, you are not inadmissible. No waiver is required.
3) If you committed a crime involving moral turpitude, the max sentence you could receive is 6 months jail. This is the petty exception rule. No waiver is required. This rule does not work with prostitution, solicitation or drug convictions however. The 6 months jail sentence is the max punishment you could receive if said crime was committed in the US. Offences that could qualify for this would be theft under or mischief under if tried summarily in Canada.
4) No waiver is required if the max sentences you received for 2 or more consecutive convictions not involving moral turpitude did not exceed 5 years improvement. This means if you were convicted 5 times for DUI and your combined sentences did not exceed 5 years imprisonment, you are no longer inadmissible.
Obviously, prostitution even though is legal in Canada is considered a crime involving moral turpitude for US immigration law. It is up to the US immigration officials to determine if your said crime is of moral turpitude.
Moral turpitude is a vague concept and many times than not assault with a weapon may not be considered a crime of moral turpitude. Other instances could be break and enter or a mischief charge which involves damaging private property.