surplus earning

Hi

I previous posted concerning a relative who was already on PIP and UC due to long-term health issues and who was going to be given ill health pension by the NHS. This has happened. She received back pay of 12 weeks holiday pay (covering the 3 year she was signed off sick). The UC sent a message saying this payment placed her
" £... over the amount where you Universal Credit payments stop. We call this surplus earnings.
This means your UC payment on 21st December 2023 will be £0
We will carry fowards the £.... as earnings when we work out your future payments. They will be reduced.
Your statement explains how we work this out"
I have looked up surplus earnings in the Disability Handbook ed 48 but I do not really understand the implications.
The surplus is around £700 how will her future payments be impacted? Is there a sliding scale of how much of this surplus earnings will be used up each month? Do they stop the benefits until the surplus is considered used up?
My relative has looked for the 'statement' on her UC account but cannot find it: is it something the UC will send her.
We rang the UC last year to say she would be getting back pay and a lump sum, the very nice guy we spoke to said that anyting under £6000 would not impact her UC. There was not mention of a cap on the amount of a one off payment beyond impacting the £6000 capital limit.
We just need some clarity on what the implications of surplus earnings are? What is the criteria they use for future payment?
This is not a disasterous situation, they have not stopped her UC completeluy -they have accepted she will never work again- but until we know the full implications we will not be able to fully explain the situation to he.
Thanks for your kind attention