From what I've read a lot of the money is to cover pensions and other work place stuff. There is also a load of agreements that we've signed that require we pay money for.
Edit: also Cameron agreed to keep paying into the EU until 2020.
https://www.instituteforgovernment.org. ... vorce-billhttps://www.channel4.com/news/factcheck ... rom-the-eu"1. Outstanding budget commitments
The EU budget operates through a multi-annual spending structure, which means projects are paid for over a period of several years. As a result, EU budget payments are back-loaded and many will be paid out post-Brexit. For example, a key element of EU spending allocations consists of cohesion fund payments, aimed at raising living standards in the 2004 Accession countries. According to the CER, only 25–30% of the biggest cohesion fund payments will actually have been spent by the time Britain is expected to leave the EU in 2019.
The EU may argue that the UK should honour the obligations it signed up to for this budget period even after we leave. The UK may seek to use our exit date as a cut-off.
2. EU officials pensions.
Like the UK civil service pension scheme, the Pension Scheme of European Officials (PESO) is an unfunded scheme and operates on a ‘pay-as-you-go basis’, with costs being covered by the annual EU budget as they arise. The EU may argue that the UK should make a payment now to cover future liabilities incurred while we were a member.
It may also argue that we should pay our share of the total cost of all European Commission staff pensions rather than simply meet the lower bill for UK nationals in the Commission – the difference coming from the under-representation of British officials. The sum could either be met out of future payments- or converted into a lump sum on departure of between €5–10 billion.
3. Contingent liabilities
The EU incurred contingent liabilities during UK membership. These liabilities effectively constitute payments that would be triggered in specific circumstances only, for example, Ukraine defaulting on its EU loan. When the 2015 EU accounts were drawn up, outstanding loans to Hungary, Ireland, Portugal and Ukraine collectively amounted to €49.5 billion. The EU may seek either a commitment from the UK to meet possible future liabilities – or an upfront payment in case they materialise in the future."
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