What creditor debt you have, what assets you have, what contributions from your surplus income you can afford, over how many years you are proposing to pay contributions and how the proposed IVA compares to that of Bankruptcy, which usually has higher costs giving a lower dividend to creditors.
In a nutshell -- they take your income and give you certain allowances on which to live and pay essential bills (expenditure) --- Income less expenditure gives them a figure for your disposable income ... this is the proposed IVA monthly payment amount. They use this as Lisa described above to see if an IVA is viable, then the creditors meet to decide if they will accept this amount or not.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014