In Canadian constitutional law the division of jurisdictions is based on legislative power being exercised within limited heads of power - for example property and civil rights. But to determine where legislation properly falls the court must consider the pith and substance of the legislation. The Supreme Court rules in Rogers Communications Inc.v.Châteauguay (City) 2016 SCC 23 as follows:
In analyzing the pith and substance of the notice of a reserve, the Court must consider both its purpose and its effects: Goodwin, at para. 21; Quebec (Attorney General) v. Canada (Attorney General), 2015 SCC 14,  1 S.C.R. 693, at para. 29; Reference re Securities Act, 2011 SCC 66,  3 S.C.R. 837, at paras. 63‑64; Quebec (Attorney General) v. Lacombe, 2010 SCC 38,  2 S.C.R. 453, at paras. 20‑22. The purpose of a municipal measure, like that of a law, is determined by examining both intrinsic evidence, such as the preamble or the general purposes stated in the resolution authorizing the measure, and extrinsic evidence, such as that of the circumstances in which the measure was adopted: Lacombe, at paras. 20‑22; COPA, at para. 18; Canadian Western Bank, at para. 27. As for the effects of a municipal measure, they are determined by considering both the legal ramifications of the words used and the practical consequences of the application of the measure: R. v. Morgentaler,  3 S.C.R. 463, at pp. 482‑83.
When conducting a pith and substance analysis, a court must avoid adopting the watertight compartments approach, which this Court has in fact rejected. The fact that a measure has what are merely incidental effects on an exclusive head of power of the other level of government does not suffice to justify declaring that measure to be ultra vires: COPA, at para. 18.
Our colleague correctly points out, at para. 85 of his reasons, that when the courts apply the various constitutional doctrines, they must take into account the principle of co‑operative federalism, which favours, where possible, the concurrent operation of statutes enacted by governments at both levels: Saskatchewan (Attorney General) v. Lemare Lake Logging Ltd., 2015 SCC 53,  3 S.C.R. 419, at para. 22, quoting Lacombe, at para. 118, per Deschamps J. (dissenting); Marine Services, at para. 50, citing General Motors of Canada Ltd. v. City National Leasing,  1 S.C.R. 641; Canadian Western Bank, at para. 37.
However, although co‑operative federalism has become a principle that the courts have invoked to provide flexibility for the interpretation and application of the constitutional doctrines relating to the division of powers, such as federal paramountcy and interjurisdictional immunity, it can neither override nor modify the division of powers itself. It cannot be seen as imposing limits on the valid exercise of legislative authority: Quebec (Attorney General) v. Canada (Attorney General), at paras. 17‑19. Nor can it support a finding that an otherwise unconstitutional law is valid. This Court commented as follows in Reference re Securities Act, at para. 62:
In summary, notwithstanding the Court's promotion of cooperative and flexible federalism, the constitutional boundaries that underlie the division of powers must be respected. The "dominant tide" of flexible federalism, however strong its pull may be, cannot sweep designated powers out to sea, nor erode the constitutional balance inherent in the Canadian federal state.