What it cannot tell you

The check is deliberately conservative. Knowing where it stops is what lets you trust the part that does not.

A clean report is not a promise of a solve

The check proves impossibility. It cannot prove possibility.

Everything it can reach came out fine. Whether FET then finds an arrangement, and how long it takes, is a different question that only solving answers.

A timetable can pass every check here and still be genuinely hard — because difficulty is about how the choices interact across thousands of lessons, not about any single count coming out short.

The difficulty number is an indicator

The report carries a difficulty score, and it says on its own face that it is an indicator rather than a verdict. Treat it as a temperature reading: useful for comparing your timetable against itself before and after a change, not a prediction of success.

A high number is a reason to look at where the pressure is, not a reason to expect failure.

Some counting depends on your solver mode

Most of the check runs in every mode. Where a lesson may legally start, what the rules leave once they are all applied, and what that implies for the lessons tied to it — a day is a day and a slot is a slot, whatever the mode.

What stops is anything that counts a week. Hours per week against the slots a week holds, how many days a subject must spread across, the gaps between lessons in a week. In Terms mode a week is not the unit it looks like, and a count read across the whole day list would say you have MORE room than you do. That is the one direction a proof must never be wrong in, so those checks are withheld rather than guessed.

The hygiene checks — dangling references, rules aimed at switched-off activities, duplicates — apply whatever mode you are in.

If a section reports fewer findings than you expected, the mode is worth checking before the rules are. See Solver modes.

Some things are absent on purpose

A rule that FET supplies for itself is not reported as missing, because it is added when the file is written. Reporting it would be a permanent false alarm about a thing that is never actually wrong.

More generally: where the check cannot decide, it says nothing rather than guessing. That silence is a deliberate choice, and it is the reason a proven-impossible finding can be trusted without verification.

It does not check your intentions

Nothing here knows what your school actually needs. A timetable can be provably possible, solve quickly, and still put every science lesson on a Friday afternoon.

The check answers can this be built. Whether it should be is still yours.

Where to go next

• What the pre-flight check does
• The four kinds of finding
• Insights — for the shape of a timetable that already exists.