Sales tax on goods is a federal matter. Sales tax on services is a provincial one, and each province administers its own regime under its own law and its own authority. The practical consequence is that a transaction has to sit on one side of that line, and both sides have an interest in where it sits.
Composite arrangements are where it gets decided: equipment supplied with installation, software supplied with configuration, goods delivered under a contract that also carries a service. How the contract is written and how the invoice is raised usually settle the answer before anybody thinks to ask the question.
Being wrong in either direction has a cost — tax accounted for to one authority and then demanded by the other, or input tax claimed against the wrong regime. Because this practice advises on both sides of the line, the question can be looked at once rather than twice.
No rate, threshold or filing date is given here. Those change, and a position on a specific transaction needs the current law and the contract itself in front of it.