What was the amendment?
The amendment would have removed the grandfathering provisions from the bill. These provisions mean that the old compliance system continues to apply to particular companies - in this case, some 1500 companies.
The grandfathering of exempted proprietary companies was done to avoid disrupting businesses which would have established themselves under other business forms, if they had known that they would have had additional compliance and reporting requirements which might be subsequently introduced.
However, those who supported Senator Muir's amendments argued that the grandfathering provisions were meant to be temporary and that the time has come to remove them (read the whole debate after 7:28pm).
These grandfathering provisions are one of the most controversial aspects of the bill.
What does the bill do?
The bill strengthens the laws against tax avoidance for certain companies. For example, it introduces anti-avoidance measures to deal with multinational companies with an annual global income of more than $A1 billion that use schemes to avoid having to pay tax in Australia or at least reduce that tax to a minimum.
To learn more about the bill, see the bills digest.