Balancing payment/POA (what does it include)?

I am a sole trader. Is the POA 50% of just income tax? Or is it 50% of income tax and NI2/NI4? Or something different? Is the balancing payment the difference between your total tax calculation (income tax, NI2, NI4) or is it just the difference between income tax for the current and previous year? And then to confirm, the January payment is balancing payment + POA 1 + student loan.

Hi,
You can see guidance here:
Payments on account
Thank you.