Two dimensions of how central tax revenue is shared under the Finance Commission's award: vertical devolution is the share of the divisible pool given by the centre to the states as a whole, while horizontal devolution is how that states' share is divided among the individual states.
The vertical (centre-to-states share) versus horizontal (among-states share) distinction and the criteria are testable Finance Commission facts; verify the latest percentages.
Vertical devolution is the centre-versus-states split of the pool; horizontal devolution is the split among states; the 41 (or 42) percent figure refers to the vertical share.
Vertical devolution is the centre-to-states share of the tax pool (41 percent under the Fifteenth Finance Commission); horizontal is its split among states by formula.