SA302 Tax which would otherwise be chargeable on non-disregarded income

Hi I am non resident and using 3rd part software to submit my self assessment The software generates SA302 Calculation. I have a small income from let property, minus my personal allowance my total income is ZERO and Tax due is ZERO. In the SA302 it shows "Tax which would otherwise be chargeable on non-disregarded income £506.40" I don't know where this figure comes from and what it means, I have asked the 3rd party software and they say its HMRC's calculation and its correct but couldn't really tell me what it is, Is it possibly the tax I would have paid if I wasn't using my personal allowance? Thanks

Hi alp,

Please have a look at the following links regarding disregarded income.
SAIM1170 - Savings and investment income: non-residents
HS300 Non-residents and investment income (2023)

Thank you.