Thank you for the question.
The purpose of the bill is to make life easier for taxpayers. If the Canada Revenue Agency uses definitions other than those of Revenu Québec, it is entirely possible for Quebec to obtain the information and transmit it to the Canada Revenue Agency. This is what is done for the harmonization of the GST and the QST. Ottawa changes the percentage of the GST as it sees fit, and Revenu Québec collects the tax according to the income, quite simply.
If the single tax return is adopted and it turns out that the definitions are different, Revenu Québec will be able to automatically convey the requested information to the Canada Revenue Agency, and Ottawa will continue to exercise full sovereignty with respect to its tax policy in Quebec.
The goal is to simplify the process by eliminating duplication of work and reducing paperwork.