die Ziffer
digit
A single written character used to represent a number, especially one of the ten Arabic numerals (0 to 9).
Die Geheimzahl besteht aus vier Ziffern.
The PIN consists of four digits.

digit
Loading your saved state.
digit
A single written character used to represent a number, especially one of the ten Arabic numerals (0 to 9).
Die Geheimzahl besteht aus vier Ziffern.
The PIN consists of four digits.

The meaning of 'Ziffer' (digit/item number) depends on the context. If it is about the building blocks of numbers, e.g. in a PIN or phone number, it means a numeral (0-9). If it is about lists, laws, or documents, it means an item number.
Meine Postleitzahl hat fünf Ziffern.
My postal code has five digits.
In connection with numerals, a distinction is often made between 'arabischen Ziffern' (Arabic numerals: 0, 1, 2...) and 'römischen Ziffern' (Roman numerals: I, V, X...). A number 'besteht aus' (consists of) several digits.
Die Jahreszahl am Gebäude ist in römischen Ziffern geschrieben.
The year on the building is written in Roman numerals.
In the meaning of 'Kennzeichnung' (identification/labeling), 'Ziffer' (item number) is often used with the preposition 'unter' to refer to a specific point in a document or list. The word then often appears without an article.
Weitere Informationen finden Sie unter Ziffer 3.2 der Anleitung.
You can find more information under item 3.2 of the instructions.
number
'die Nummer' and 'die Ziffer' both have to do with numbers, but they do not mean the same thing. 'die Ziffer' is usually a single character such as 0, 1, or 9. 'die Nummer' is often a whole sequence of digits that identifies something or someone, for example a telephone number or customer number. They can be confused because every number can consist of digits. Memory tip: A digit is a single character; a number is usually the whole identifier.

In der Postleitzahl fehlt eine Ziffer, deshalb kommt der Brief zurück.
A digit is missing from the postal code, so the letter comes back.
Herr Richter schreibt die Kundennummer in die E-Mail.
Mr. Richter writes the customer number in the email.