As the "yes to independence" campaign is launched in Scotland, Channel 4 News looks at what it would mean for the UK and why the SNP seems reluctant to use the word "separation".

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On the morning of the campaign launch urging Scots to vote for independence, Scotland's Deputy First Minister Nicola Sturgeon chose her words carefully during a radio interview: "It's not about breaking up, it's not about separation - we can continue to share things where that makes sense," she said.

Why then bother with independence at all? And what impact would it have if the SNP's model is based on sharing?

The decision not to name the elephant in the room is partly to do with winning votes - this is politics after all. A poll on Friday found that despite the SNP being Scotland's biggest party, only a third of Scots actually want independence, so the SNP will be trying its best not to frighten voters with assurances that not too much will change.

But the SNP also points out that there are many models of independent states which are mutually reliant. For example, countries in the eurozone are certainly independent, but are also inter-dependent through a monetary union. The SNP want some kind of similar relationship with the UK - fiscal independence but within the shared sterling currency.

'Reserved areas'

But as a devolved government, Scotland's Holyrood parliament already has control over education, health, justice and transport, for example, so many of these areas will not involve much more "sharing" post-independence (something that Scottish Labour is at pains to point out).

The forthcoming 2014 referendum means that the SNP are having to spell out their version of independence in a way they have never done in the past. Professor James Mitchell

It also means that the SNP has already been able to develop policy on these issues now, and post-independence, and the only difference post-independence will be the ability to levy more funding from taxes and budgetary decisions.

But it is the "reserved" areas of defence, foreign affairs, macro-economic policy and overseas aid, all of which are still controlled by Westminster and shared across the UK, where post-independence financing and policy - "shared" or not - is more uncertain.

Defence

Ever since defence secretary Philip Hammond called the SNP's defence policy "laughable", the party has been at pains to come up with a strategy for how defence could be divided.

There are currently around 3,500 to 4,000 British soldiers in Scotland, across five bases, one royal navy base and one royal air force base and another 6,500 in Faslane on the River Clyde, home to the UK's nuclear submarines, however the numbers vary widely depending on ongoing operations.

The SNP wants its own Scottish defence force, but as an MoD spokesman told Channel 4 News, it is not so easy to divide up existing army personnel: "It's the British army, regardless of what country you are based in, so units will rotate, move around depending on demand."

Spending and security

Wider issues of defence spending and the implications on the rest of the UK are even more complicated when considering a separation. Trident nuclear submarines are currently in Scottish waters and removing these weapons, as the SNP says it wants to do, as well as developing its own policy on Trident, could be one of the hardest issues the Scottish government would have to grapple with.

It's the British army, regardless of what country you are based in, so units will rotate, move around depending on demand. MoD

Critics of independence say Scotland could not afford to fund its own separate defence industry, and that this will undoubtedly have implications for security and spending throughout the UK.

But then that is in comparison with the sum of 2.7 per cent of GDP forked out by the UK. In his report on defence in an independent Scotland Malcolm Chalmers, director of defence policy studies at the Royal United Services Institute, says it is unlikely the Scotland would choose to give defence such a high priority.

Welfare spending

As a result, it may be able to spend more on social welfare, for example. The current fund per head is between 9 and 13 per cent higher in Scotland than in England, depending on which measure is used.

"That 9 per cent equates to around £1bn in total - that's something you'd need to find and it might come from savings elsewhere in the budget," Professor David Bell, an economist at Stirling Univerity, told Channel 4 News. "That's why defence would become a very interesting issue. It seems that maybe the implication would be that Scotland spends less on defence post-independence."

And when it comes down to a vote, one of the main factors may well be the protection of the welfare state, Professor James Mitchell, nationalist politics expert at Strathclyde University, told Channel 4 News.

"Our research showed that people's vote for devolution in 1997 was about creating a protective mechanism - it was a defensive move to protect the welfare state that they felt was under attack," he said.

"If Scots come to think the British future is the decline of the welfare state, that could play into the debate."

North Sea Oil

The Scottish government's trump card is North Sea oil. As Factcheck has previously pointed out, "detailed research of the money brought in by the UK oil and gas fields suggests that Scottish waters accounted for 91.1 per cent of UK North Sea revenue in 2008/09."

This is obviously hugely dependent on the price of oil, but in 2008/09 the money guaranteed from this source soared to nearly £13bn, which contributed to Scotland' budget surplus at the time.

Many Scots nationalists say that despite higher spending per head in Scotland, the revenue from North Sea Oil would more than compensate for a lack of funding from central government, especially if they could also levy their own taxes and save in other areas of spending.

But then, post-independence, would the North Sea waters be considered solely "Scottish" and will Scotland and the rest of the UK agree?

As the "yes to independence" campaign kicks off, the unanswered questions are only just beginning to emerge. Or as Professor Mitchell puts it: "The forthcoming 2014 referendum means that the SNP are having to spell out their version of independence in a way they have never done in the past."

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