The capital BG is the wrong way to write it, and the correct way to write it is GB, which refers to the national standard. GB/T refers to the recommended national standard (GB/T), which you can read and recommend here. Recommended national standards refer to national standards that are voluntarily adopted through economic means or market regulation in production, exchange, use, etc.

Any unit has the right to decide whether to adopt such standards. Any unit that violates such standards will not bear financial or legal responsibility. However, once accepted and adopted, or all parties agree to incorporate it into an economic contract, it becomes a technical basis that all parties must abide by and is legally binding. GB usually refers to mandatory. If some of the contents are not mandatory, there will also be explanations.