Many people refer to Title II firearms (Silencers, SBR, SBS, and Machine Guns) as Class 3 weapons) as Class 3 firearms or weapons. The Class 3 terminology is generally used with a dealer license and not a type of weapon but a license. SOT means special occupational tax and refers to the tax due upon transfers of items restricted by the NFA ( Generally $200 except for AOWs which have a tax of only $5)
If you purchase a Title II firearm from a dealer or individual out of your state, you will need to use a class 3 transfer agent to move the items across state lines. If you purchase from an individual within your state then no transfer agent is required.