Register: The Same Facts, Three Different Documents

The same information, three ways:

The system’s down again.
The system has failed for the second time this week.
A second service interruption has been recorded within the current reporting period.

None is more correct than the others. Each is right in one place and wrong in the other two.

What actually changes

Vocabulary. Anglo-Saxon words read as plain; Latin and French borrowings read as formal. Buy against purchase, start against commence, get against obtain.
Contractions. Don’t, we’ll, it’s mark informality. Their absence is one of the strongest formality signals in English.
Sentence shape. Formal writing runs longer and subordinates more. Informal writing is shorter and more coordinated.
Distance. You and I close the gap; the passive and nominalisation widen it, often to nobody at all.

The Anglo-Saxon and Latin pairs

English has two vocabularies for many concepts, one from Old English and one arriving with the Norman conquest. The Anglo-Saxon word is usually shorter and warmer:

ask / enquire · end / terminate · help / assist · show / demonstrate · use / utilise · need / require · tell / inform · let / permit

Choosing consistently from one column is most of what people mean by tone. Mixing them at random is what makes writing feel unsettled.

The two ways to get it wrong

Too formal for the situation. This is the more common error and the less noticed, because it feels safe. It costs warmth, and in an email it can read as cold or evasive. Please be advised that your request has been received says less, and less kindly, than I’ve got your request.

Too informal for the situation. Costs credibility, and it is immediate. Contractions and casual vocabulary in a legal submission or an academic paper mark the writer as not knowing the conventions of the room.

Reading the room

Three questions settle it faster than any rule:

Who will read this, and do I know them? Distance in the relationship maps directly onto distance in the language.
What is the document for? A record wants precision; a request wants warmth.
What register did they use? In correspondence, matching the other person’s level is almost always right.

The thing formality is not

Formal does not mean vague, and this is the confusion that produces most bad professional writing.

It has been determined that a delay may be anticipated is formal and says nothing. The shipment will arrive on Thursday, two days late is equally formal and says something. The first is not more professional; it is less useful, and readers can tell.