This reference is for employees that don’t have a P45 tax form. This is filled out if you start a new job and you don’t have your P45 available or are starting a second job.
Every employee will receive this at the end of each tax year. You get a separate P60 for each of your jobs.
This is given to employees when they stop working at a company. P45 is the reference code of form with details of termination of employment.
Information sourced from the Government website