Splitting your tax code is useful in avoiding overpaid tax if you have more than one source of income, factor, and both of them are below the personal allowance (currently £7,475).
Let’s say I have job number one which brings in £5,000 and job number two bringing in £4,000. On earnings of £9,000 the tax due is 20% of my pay above £7,475 (which is £305).
If I give the first employer a tax code covering my full personal allowance (745) I will pay no tax via that job. But my second employer will deduct 20% from all my pay (he will use the “BR” code as outlined by buildersmate) so I will pay £800. This leaves me the hassle of getting£495 back some time the following summer.
If I split my code, I get the taxman to provide a code of 500 to employer No. 1, covering my £5k. He gives employer No.2 a code of 247 (the balance of my allowance). He therefore only deducts 20% tax from (£4,000 - £2,470) which amounts to £306. (The £1 discrepancy arises because the tax codes are in multiples of £10 and the personal allowance has an odd £5).
It’s only of use when both incomes are below the personal allowance. If any one of them exceeds this tax is due on all of the second at 20%, so there is no point.
Like you I am inclined to think that there may not be an issue for annieigma’s daughter.