Who gets to be a citizen and how do you set up immigration rules?
Who becomes a citizen of an independent Scotland?
This is two different questions. Citizenship laws can be changed by different governments when they are elected once Scotland is independent – but there needs to be a way of defining who is a citizen at the point of independence. Anyone who has UK citizenship and is habitually resident here would get automatic citizenship – as will any of their children when they are born (even if only one parent is a Scottish citizen). Anyone who has a parent or grandparent who is eligible to be a Scottish citizen would not have automatic right to citizenship but an automatic right to apply and this could start during the transition process to independence. This could also apply to migrant workers currently in the country and students studying here at the point of independence – but that is more of a policy decision. Some initial rules would need to be set for the naturalisation process for those who applied before a post-independence government was elected (at which point all these decisions would pass to the Scottish Parliament).
What about immigration?
While this is a domestic policy matter that would be handled by whatever governments are elected after independence, there shouldn't be a period where immigration is impossible so the infrastructure of an immigration system and a base-level process of naturalisation should be in place to ensure continuity.
What about Scots' current UK citizenship?
This can't be decided by Scotland alone but Scotland would allow dual citizenship for anyone who wanted it – subject to the UK accepting that.
So what would happen for citizens?
The National Insurance Number is the de facto 'citizen identifier' for the UK but it is a UK number and so can't be used to identify Scottish citizens. So all the new Scottish citizens would be provided with a Citizen Identifier, a number which is unique to them which they can use in a manner similar to the way they currently use a National Insurance Number. They would also get to apply for a Scottish passport . There is an international standard for passports so there is no real complication in making passports universally accepted.