Why is PRSA both a charge and a relief on Form 11

Can anyone explain the following;

I have a PRSA into which I make the majority of the contribution through payroll. My employer also contributes but only a relatively small amount.

Why on Form 11 do I have to put the employer contribution e.g. 500 on the taxable benefits section (in the PAYE/BIK/Pensions screen) and then later on I put the same 500 amount in the PRSA sections (in the Charges & Deductions screen) and it calculates that I am due relief on 500.

Which is it? - a tax or a relief.........

Because a PRSA contribution made by an employer is a taxable benefit in kind. You only get relief up to the relevant age related contribution limit. For example, if the maximum you can contribute is 15% of your salary, and you contribute the full 15%, any contribution made by your employer will be taxable as income, with no relief.