Tbol is C17 AA, or it is alpha alkylated at carbon 17. This is a major distinction among oral steroids, and is actually what separates them as a class from something like proviron, which we would not consider an "oral" because it does NOT have this methyl group added at carbon 17. This added methyl group allows the oral steroid to somewhat bypass the liver and first pass metabolism and get into your bloodstream. This stresses the liver, as it has mechanisms which if it fails to metabolize something, tbol in this case, it releases enzymes and what not in order to attempt to metabolize this drug faster. This is essentially why liver enzymes raise while on oral steroids, and which is why TUDCA is recommended.
The remaining 3 characters in this field are for the optional input of a Part identity and which allows several Export Declarations to have the same core DUCR . It is this, in combination with the (up to) 35 character UCR which must be unique. An example of Part identity use would be a large consignment covered by a single DUCR but being exported on say 3 vessels over a period of weeks or months. Ideally, the DUCR would use a common identifier in the trader’s records, but each movement of the goods would have a separate CHIEF declaration, using the same DUCR but designated as Part 001, 002 and 003.