It's in the link I gave you.
'The numbers in your tax code tell your employer or pension provider how much tax-free income you get in that tax year.'
So it is not the 'tax relief'. To be precise, it is the amount of tax-free income you will receive before income tax is applied at your...
As an aside, the tax year before last, I submitted my tax return early in December, well before the Jan 31st deadline. My pension provider informed me that HMRC had made an - extremely - adverse change to my tax code, between Christmas and New Year, to take predicted tax owed well in advance...
I've never used that function, but all that makes sense. You get tax relief on private pension contributions, so if you want to receive that tax relief through your tax code, that is how you would do it. You would see the figure of tax free income increase.