For example: If you submitted a year to date figure of pay of £21,000 (value A) on your last FPS for the year 2015-16 but realised this should have been £20,000 (value B), then you must enter £ -1,000 ( value C = B-A ) at this stage.
Please note that your EYU should only record the difference between what you had originally submitted and what the figure should be. If you need to submit more than one EYU, each submission must only show the amount of the amendment from the last figures submitted.
More information can be found here : http://www.hmrc.gov.uk/payerti/getting-started/payroll-summaries/eyu.htm
The EYU data provided by the system can be edited to any figures you are happy with. The system will only generate the EYU data for you if you had completed the final pay run for the previous tax year but missed the 19th April deadline, or you had e-filed the month 12 or week 52 FPS but you did a roll back after that to rectify the errors.