That's the nationalist contention, which they base on the portion of the oil and gas fields that would belong to a hypothetical Scottish state, which would een by the meanest estimate have claim to the majority of it.
The unionist contention is that the fields are the property of the Union, collectively, and that Scotland's share is proportionate to its share of the British population.
You can make a case for both, and neither is obviously superior to the other, so in practice the nationalists fall back on the observation that Westminster has pissed away the oil revenues to the benefit of neither Scotland particularly nor Britain generally, and the unionists fall back on, well, pretending not to have heard the question, basically.