If we want to pass SBSA level6, do we use the latest ACS tool and specify the level6 test, or do we use the 6.1.0 tool to run the level6 test?
It is recommended that you use the latest tagged version of th tools for the level you require
For your question, you are asking about the requirements for level 6
for the SBSA it is recommended you use the 7.1.0 tool (latest at time of writing)
For the BSA tests please also use the latest tagged version. At teh time of writing this would be the v23.01 REL 1.0.3 version of the BSA ACS
For certification commands it is required to run the SystemReady ACS (github.com/.../arm-systemready) which will automatically take care of the commands to run both the SBSA and BSA ACS tests