ASC45 form is issued when you leave a public service job. It lists your salary and the amount of ASC you have paid from the 1st January to your cessation date for this employment.
If you are starting a new job also in the public service a copy of the ASC45 from the old job should be sent to the payroll department of your new employment.
This will ensure that you can be taken off the Emergency basis of ASC which you will be on when set up in your new job.