Tankerman assistant or Basic oil and chemical tanker cargo operation

Can anyone tell me the difference? After reading the certificate I think BOCTCO is a sign off of the master saying you are qualified to supervise the transfer operation. Not to sure on this one as at the end of the cirt it talks about advanced so.

See 46 CFR 13.401 and 13.609, and also NVIC 22-14. The attestation by the Chief Mate or Master is for STCW Advanced endorsements. For STCW Basic, you either need the course for Tankerman-Assistant or the examination for Tankerman-Assistant. If you meet the requirements for Tankerman-Assistant you also will meet the requirements for the STCW Basic endorsement.