Check 234A, 234B and 234C against a notice. Recompute the three lines from filing date, credits and advance-tax payments, and see which reading fits.
Why does this page ask for the date section 234B was charged to?
Because section 234B keeps running month by month until the unpaid balance is treated as paid. A notice figure without that end date is only half a working. This page makes you name the date, because otherwise it would have to guess the one fact that decides the line.
Why does this page not ask for self-assessment payments one by one?
Because it checks the common one-date path only - the remaining tax paid on one date, or the notice charged through one date. Staggered self-assessment payments change section 234B month by month, and this page says out loud that it is not quoting that branch.
What if my return was never filed?
Then this page is too small on purpose. Section 234A has a no-return branch that runs to best-judgment assessment, and this page does not quote it. It checks the common filed-late path only: one due date, one filing date, one notice figure.
Why do sections 234B and 234C both appear?
Because they ask different questions. Section 234C is about being short at one instalment date inside the year. Section 234B is about reaching the end of the year having paid less than 90% of assessed tax in advance. The same notice can lawfully carry both.
Why does this page talk about a tolerance under section 234C?
Because the statute does. Pay at least 12% by 15 June or 36% by 15 September and section 234C does not charge anything at those two dates even if the instalment itself was short. Many summaries miss that, and the notice figure then looks higher than the law really says.